For later Kups, one of the belt test items is sparring against two opponents at the same time.
What is being tested with this, and what would be a proper tactic to pass it?
When there are two or more opponents to fight, they will easily win if they can gang up on you and take you on all at the same time. They want to overwhelm you by striking you from all directions, making it impossible for you to defend yourself. So that's their primary strategy and what you must avoid letting them do.
Your training must involve learning to move in such a way that prevents more than one person from engaging you at any given moment. Don't let more than one person get within striking distance of you.
The primary way of accomplishing this is to move such that your opponents are arranged in a line, with one of them in front, and the rest in back. You'll have to move very quickly and be able to see not just the person that's in front of you but everyone else as well at the same time. That's really what this is about: situational awareness and strategic maneuvering.
If you get too focused on the opponent in front of you, you can lose sight of what's going on around you. Another opponent can then use that to his advantage. So don't become too invested in the person you're fighting at any given moment. This isn't about winning. This is about surviving.
Be quick to change from one opponent to another. The group will try to outflank you from both sides. Quickly run to the opponent on the outside of one of the flanks. As the group sees your pattern, they might adjust to other strategies. Be ready to adapt.
Another thing to consider is who you engage. The worst opponent to engage is the one that's the strongest. Doing so will take more effort to deal with than any of the others, and that puts you at greater risk of not being able to adapt to the group's movements. So you strategize and try to target the weakest opponents first, and use them as shields from the strongest ones. There will be times you have to take on the strongest one, but you can minimize that time and quickly change to one of the other opponent's when an opportunity presents itself. Don't wait for the opportunity. Make one yourself.
It forces you to remain on your toes, always moving and adapting to the group's movement.
You need to consider your surroundings as well. This is your school, of course. But you have walls. You have posts that go from floor to ceiling. I wouldn't expect other things like chairs, tables, doors, and so on. But if you ever were to take this practice to another level, it would be to add these kinds of objects into your practice environment.
These obstacles can be used to your advantage. You can put yourself up against a wall, for example, to ensure that nobody can get behind you. It limits how many people can engage you at the same time. And it limits your opponent's mobility and maneuverability. Of course, it also limits what you can do as well, but maybe you can practice that particular scenario ahead of time and get very good at it. When it happens in real life, you might be able to use that skill to your advantage. People that practice close-range fighting, such as Wing Chun, are much better at this.
In real life, tables and chairs can be moved around and toppled to block your opponent's path. You can pick up objects and throw them at your opponents in order to cause them to stop and move in the other direction.
The primary goal in real life should be to maneuver yourself and your opponents such that you have a safe way to exit the scene. If you're in a bar, for example, and your opponents block the way to the only exit, then you need to move them around to the opposite side of you. Use the tables. Have them come to you, then circle around the tables, and once you have a clear path to the exit, run away.
Some martial arts practice a kind of play acting whereby if your opponent gets hit, they might pretend to be hurt and drop to the floor temporarily. But in styles like Taekwondo, which is what I think you're talking about, opponents don't stop. They just keep coming. This is maybe a bit more practical and realistic, because in real life, you risk being overtaken by the horde of people if you get into a scuffle with any one of them. Your best bet is to limit your contact and maximize your maneuvering. Only go for a KO strike if it doesn't put you at risk of someone grabbing you and holding on.
Which brings me to the next issue: Grappling. If someone gets a hold of you, your mobility is seriously compromised. There are things you need to know how to do in this scenario. You can still move yourself and use your opponent as a shield. Your primary goal should be to escape the hold and regain your mobility as quickly as possible. The only way to do that is to understand and be good at grappling.
Lastly, I just want to say that nobody is particularly good at fighting more than one person at a time for real. If you were to practice it "realistically", you'd see even your best, highest ranked fighters in your style fail at it more often than not.
You can manage to convince yourself that you're good at it, because you do well in class against your fellow students. But your classmates are being sort of nice to you. They're not really attacking you like you'd see in real life. If you really asked them to go all out and do anything, even stuff that's against the "rules" in your martial art, you'd be in for a surprise. Better yet, get some MMA guys to fight you at the same time. It's really going to be enlightening.
But that doesn't mean it's useless. It's just there to give you the fundamental concepts of positional strategy and maneuvering. Hopefully you can use it to survive, even if you don't come out unscathed. If you increase the realism level of your practice over time, when you're ready for it, you'll be better prepared for it if it happens in real life.
Hope that helps.
There can be many purposes, it depends on the instructor's plan at the moment.
Generally, your goal is to position yourself so that you spar/fight only one person at a time. That is the most efficient way to handle multiple opponents.
Your instructor may not like how you manage your footwork and positioning with a single opponent. Perhaps, you don't move; or maybe you only move back and forth. By having additional opponents, you are forced to move laterally in all of the 8 primary directions.
Maybe the instructor is wary that you are getting used to your opponents, and so, by adding an additional, you are forced to not depend on the uniqueness of any one person in a class.
Maybe the instructor doesn't like how you block or parry where you could be exposed to the effects of a follow-up, a combination, or a feint. If there are no advanced students who can aggressively issue the follow-up, combo, or feint, that can artificially be done with a second opponent.
Maybe the instructor is conditioning you up for tougher opponents. By pairing you off, you both equally expend energy; by introducing a second opponent, the two opponents can tag-team each other so that one can get a break while the other takes over. You, on the other hand, must always be ready no matter which one is sparring with you.
Maybe you are far superior to your classmates; in this case, adding more lesser experienced opponents sort of evens out the level of quality somewhat.
As mentioned, you should not expect to be victorious. Your goal is to survive - not win. It's not particularly fair to pair 2v1, and so, those in the team should be aware that no matter the instructor's goals, it is a training exercise. They should be training you, not pummeling you. Such sparring can be extremely dangerous. Two opponents attacking at exactly the same time is not realistic in a sport context, so, if sport is the context, then the opponents must be careful to issue tactics that approximate that. One who strikes at the abdomen while the other strikes moments later at the head can be a simulation of a double kick - one mid level, the other high level. However, both striking simultaneously to the head is not realistic in a sport context.