or the 2 sides could just chat and discuss what happens in a particular event .... i.e side A invades a planet occupied by side B.
example ahead:
the heads of side A and B chat and discuss what battles will occur and what happens.... for example: Side A wins the initial battle on landing and advances x distance. Side B counterattacks at that point and manages to push back side A y distance. both sides regroup then have one huge battle at point z which side b ends up winning and forces side a to retreat.
playing a game to decide it..... while feasible really depends on the ability of the player. not to mention any arguments about what to play to decide it.