Multi bots are when two bots have the combined weight of their opponent. They're usually built with the intention of flustering the enemy, since the Ai usually attacks the bot which most recently attacks it,
For example, say Bot A and C are a team, their opponent is Bot B. Bot A attacks Bot B, but then Bot C comes in and attacks Bot B, Bot B turns it's attention to Bot C, leaving Bot A free to attack again, and continue the cycle.
Now that you know how one works, let's see how they're built
for example, let's say there's a MW tourney
One could enter two 200KG bots, or maybe one 100KG bot and a 300KG bot. But it's more commonly two 200KG's.
the reason why is, the best combination for a Multibot is two VS', having one wedge and a HS won't work because the HS will hit the wedge/punching bag, flippers can't because they need both their flippers combined to flip their opponent. VS' or maybe drums have precise weapons with a small range, which reduces the chances of them hitting their own team, plus, it isn't mostly power orientated.
So, for the sake of being lazy, most builders opt to just have two of the same bot. However there are exceptions.