Not only is it possible for two good forces to be in conflict- I'm going to ignore the obvious (and valid) possibility of "one force is evil but just thinks they are good har har"- it is sometimes unrealistic for them NOT to be in conflict.
A caveat: Some campaigns posit the good vs. evil conflict to be the primary universal conflict, in which case there might be exceptions, but consider the following scenario:
Nation A and nation B are neighbors and both are good. Crossing the border is a river, running from A through B to the sea.
As nation A's population grows, it uses more of the water for agriculture. Eventually only a trickle is going to nation B. B tries negotiating, but A is like, "Hey, it's our water until it's on your territory, and if we give you more, our people starve."
Nation B feels justified in going to war against nation A because otherwise their people starve.
Both are good, and they must needs go to war.