Select an Angel follower in your cemetery and enemy follower on the field. Put them into their owners' EX areas.
QMy EX area, my opponent's EX area, or both are full when I play this spell. What happens?
AIf your opponent's EX area is full, the selected enemy follower will remain on the field.
If your EX area is full, the selected Angel follower will remain in your cemetery.
QCan I only play this spell if I can select both an Angel follower in my cemetery and an enemy follower on the field?
AYes, that's correct.