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.