Is it possible to effectively train in two (or perhaps even more) martial arts/combat sports at the same time? What are ways to minimize concerns like technique and muscle memory confusion? Does it help to have varying experiences in the various arts?
It's possible, and the dangers are entirely dependent on which two arts and which two teachers.
I study Isshinryu Karate. I also studied iaido for a few years while in my pre-black belt years. After about a year of the cross-training, I noticed that the iaido training had affected my empty-hand training in subtle but negative ways, such as the way my weight was shifted in stances, and it took me a while to fix it and create a proper mental separation between the skills I was learning that were specific to the purpose of handling a sword, and the skills I was learning that were specific to the purpose of working empty-handed.
As far as techniques to avoid confusion, try to create as many memory markers as possible, like the smell of the school, the equipment you wear, the way you bow (or don't). These, of course, would become Bad Things(tm) if you intended on using your skills outside for self-defense, because it creates a layer of separation between you and the skills, but by the time you're ready to use them, you should be properly aware of that anyway.
In the good old days before the fragmentation of the 'original' martial arts ways into all the 'different schools' There was really not this style or that - it wasn't stand and be counted - or lay down in your pyjamas and hug people. The martial 'arts' were complete systems in and of themselves. The old masters took on very few students. They were very carefully chosen. They were trained upright AND laying down. They were living in a world of constant war and danger (sounds like about now 'eh) "Know you enemy" was a simple but profound edict. Your enemy may be a pugilist, a grappler, an oarsman, a farmer, brigand, mad monk, etc, etc. He may have a sword, a spear, a halberd, an oar, a club, etc, etc.
The fragmentation came about when old masters passed away and families fractured and members went their separate ways. We became aware of this in Australia. In 1958 we had the Polish man Slagowski in Blacktown roughing it out with many pugilists, wrestlers,bouncers,policemen, prison guards, and brawlers. In the early 1960's We had little Merv Oakly return from Japan with Go Ju Kai (came from the Go Ju Ryu systems of Okinowa to Japan. In 1966 we had Tino Ceberano in Melbourne doing Go Ju Kai. We had many people of kyu grades leave these schools and start up there own way. We then had Richard Bradford (founded Goshin Ryu). We also had Dan grades like Bob Jones (created Zen Do Kai) - then came Go Shin Kan and Go Kan Ryu (Door Knocker Karate). Many styles many ways - some very effective others not so.
An old Okinowa master asked of prospective students (masters of other schools, ways,countries) What they did for a living - what was their profession. If they answered that their school was their profession he would dump them. You must have a separate means of funding your life and household. If you get money solely from your school you will be sorely tempted to adopt more students so that you could prosper and survive. This seems to be the modern institutional system, or way where you are guaranteed a black belt within 12-15 months !!!! More students - bigger school - bigger school - bigger reputation - bigger rep - more schools ad infinitum. Hence we have the stand ups versus the grapplers. The kickboxers versus the boxers. etc. etc. Now we have UFC, MMA, ABC etc. etc. Where EVERYONE is yearning for the COMPLETE system. You may indeed have to train in 2 or more separate martial arts these days to find a completeness - a wholeness in your martial arts. People nowadays even want to belittle, cheapen, or even negate the 'ART' part of Martial arts. The 'martial' is the fighting for survival part of it. The 'art' part is the ability to heal what has been damaged or redress the balance.
In the 'pure' Tai Chi (supreme ultimate pole - modern day is Chuan or fist) the practitioner is an observer of all that is. When other 'martial artists' see a conflict or an enemy approaching they adopt the force to force principles or right is might!!!! Sadly a mistaken assumption - they will have already lost because they have enmeshed themselves with 'the opponents' inner demons. The Tai Chi practitioner does not see a 'confrontation' they see only that the other person is out of balance - they are unwell - or are beset by some inner demons. The Tai Chi person then activates the 'art' system of healing and rebalancing. They become smoke to the other persons mirror. They do not become the mirror THEY BECOME THE OTHER PERSON (borrow their congested and/ or leaking energy) they return to their own source and then release that 'borrowed' energy back into the unbalanced or imbalanced person. This puts congested energy and leaking energy through the yin - yang tumbler and restores it to a balanced flow. The person then no longer seeks a 'confrontation' with themselves by the use of LATERAL VIOLENCE !!!!! (taking it out on your fellows - family - friends etc.). The future comes soon where all 'ways' - 'schools' 'styles' and 'systems will blend once more into the one true way of the MARTIAL ARTS !!!!
I have done multiple martial arts at the same time and it just takes a bit longer to achieve the same skill level if you learning dissimilar arts at the same time.
Too be honest I believe the best progression is made when your arts are similar or very dissimilar. Lets say you do boxing and grappling then the chances of your boxing being influenced by grappling is less than say doing boxing and Shaolin Kung Fu, since you are learning completely different techniques - punching vs grappling. The same if you are doing wrestling and BJJ for instance you might also have less of an issue than say doing Wing Chun and Boxing as they teach the same concepts but in very different ways.
I know in this modern world of MMA and instant gratification we do not dedicate ourselves to one thing anymore, but my advice is; Start with one art and work on it until you are comfortable with it and then move on to a 2nd art.
Yes it is possible without causing any problems and it is very good if the styles complement each other because they would fill in gaps you might have from one style.
For instance a striking are will be a very useful addition to a grappling art. If you are a beginner it may be a bit hard at first because there is a lot of information to take in especially if you are training both during the same day.
I'm actually doing this (Kalari and Wushu).
Yes of course. In my experience it takes about 2 months to get used to it mentally and physically. You can get over the psychical exhaustion in a about two weeks. The mental part (muscle memory) takes longer.
It would be like learning two new musical instruments. If you have had experience with music before it can be a lot easier to do it.
If you want to learn more than two fighting arts however I think you should consider MMA.
I think motor memory has two phases of learning - short term but fast learning phase where neural processes (or motor maps) are formed to perform a set of motor actions. Followed by a longer term (slow learning phase) where new modifications are made on specific motor modules to improve efficiency.
What helped me is understanding common patterns in your fighting skill. This involves breaking up a sequence to understand the basics of a move (e.g How to perform a certain block) and then perform it over and over till you get the hang of it. Once you've got it internalized, including that move in an elaborate sequence will be a lot easy.
This will entirely depend on what you want to get out of it. Are you in it competitively? Is it just for self defense? Your choice in the martial arts you learn depends on what you fancy really.
Learning to wield different kinds of weapons and the attaining the skills associated with it motivates me. I've never had interest in sparring or actual combat.
This is similar to choosing courses at college. Will they help you in real life? Is it worth your investment in time/money?. How much time are you willing to put into it? Are your choices dissimilar (eg some grappling art like jodo - combined with style changquan or some soft taichi combined with hard style muay thai)
PS: It is better you tell both instructors that you are training in another discipline at the same time. Some of them are competitive and don't like that.