battle Groups would be cool, but what I'm thinking is have one fleet attack from one side while another comes in from behind. The fleet attacking from behind being from a different sector then the one attacking from the front. This would allow for better flanking actions and more militaristic tactics.
Chewy, you're asking for tactical combat to be tactical
It would be cool to come in from both sides, I can see my buddies face as I take out all his support ships in the first round of combat!
Chewy, I've noticed when two AI controlled fleets attack from different sectors, the strat combat screen will show the fleets on the appropriate sides of the screen. So, you may be able to do this if you attack at the same time from seperate sectors.
If you're playing simultaneous turn resolution, then maneuvering 2 fleets so that they use up a move point in the same 'phase' of the turn to enter combat would work.
If you're working in tactical combat / turn by turn play, I'm not sure that you can. could try shift clicking 2 fleets then giving the order to move? No idea if that would work though.