This situation happened in the last session I ran:
Evil wizard casts a Wall of Fire across the middle of a room. She's on one side, and the PCs are on the other. The party wizard decides he's going to cast a Cone of Cold through the Wall of Fire. My gut tells me this shouldn't work, but there's nothing in the rules that says it doesn't. People/objects can pass through the Wall of Fire (and take a bunch of damage doing so), so I guess a spell could also pass through?
Can a Cone of Cold pass through a Wall of Fire?