This would very useful if a person cannot defeat a planet/ship on their own.

Basically, say a teammate and you decide to attack a planet, when you click on their ship you have an extra option, "Combined Attack". The other player would then receive a message asking if they wish to participate in the attack. If they accept, they cannot move from the sector, and the person who asked to combine their attack can then decide who to attack, and the fighter bay, torps, and beams are combined.

The problem with this is that more fighters or beams will target the weaker ship, making it more susceptible to damage.

I don't think this is ever going to happen. There are too many problems with implementing it and possible exploitation. If a player wants to take a planet with a teammate they will have to do round robin style attacks on it.

If it was used, I'd say it would have to make the two players a lot more suceptible to bounties, or else they'd be crushing the strongholds of smaller players left and right.