I really want a move that I created called Double Trouble to become a reality but I doubt it will. The move works in Double battles and basically the user waits for its partner to use a damaging attack the same turn and both attack the opponent’s Pokémon at twice the power. So if Minun used Double Trouble and its partner Venipede used Steamroller then both Pokémon end up using the move at the same time and doubling any damage given. It’s sort of like Parental Bond except it works only in Double battles and two Pokémon must be on the same side for it to work successfully. It also won’t work for moves that must charge before a successful hit or one that recharges afterwards like Giga Impact. If a multi turn attack is used then this move would use that attack for the first turn and fail if used consecutively until the multi turn attack cycle ends. This is just to keep the move from being so broken.
I think the idea is nice. However, I think despite your precaution, it might still be too OP. Now, I just want to make it clear that I have no idea which Pokémon would get access to that kind of move, and that a move is only broken or overpowered when it can be used by a Pokémon that makes it be.
I also assume that "doubling the power" was an abuse of vocabulary on your part, and that in reality, what would happen in your scenario is first, Venipede uses Steamroller, and second, Minun uses Steamroller. In practice, that's the same as "attacking at the same time" (with, I assume, the original user going "first" for logisitic programming). I don't think you meant it in a way that only Venipede would use Steamroller, but it would have twice the BP, since you mentionned a similarity to Parental Bond.
My last assumption is that it would have to be a priority move, in the same mold as Follow me, Helping hand, etc., for it to work.
Knowing this, this is where my main issue arises. If you pair a Pokemon with that move alongside a fast/scarfed strong Pokémon, you can basically hit twice before any of your opponent can move. This would be particularly devasting if the Pokemon learning Double Trouble happen to have decent offensive stats, and/or ways to boost themselves, which is likely to happen. I think we can all agree that Minun is pretty bad, but it also has access to Nasty Plot. Just imagine virtually priority thunderbolts for both on electric terrain (I say virtually due to Koko's speed tier and the possibility to have it wear scarf). This also allows very slow Pokémon (which are generally either bulkier or stronger, or both) to leapfrog their way in front of much faster Pokémon, without having any negative consequence like Trick Room would have on its faster teammate.
If anything, I think the way to balance such a move would simply be by making it a Mirror Move counterpart, which fails when used on an enemy. Basically, it would copy the last move used by the allied target. No "waiting for X to move". It would fail if the target hasn't used a move since switching in.
Other solutions to balance it, and keep the principle of attacking at the "same time", would be :
- The BP of the move used by the Pokémon using Double Trouble is reduced by 50%, 66% or 75% (the latter is in line with Parental Bond, but I think it would be too much)
- The move doesn't have priority, and it only works if used before the targeted allied moves (honestly, this might be what you meant all along, so sorry for my initial assumption if that's the case!)