You choose which mode you’re using—or that you’re using both modes—as you’re casting the spell. Once this choice is made, it can’t be changed later while the spell is on the stack.

If you choose both modes, they can each target the same creature or they can target different creatures.

Winterflame won’t affect any target that’s illegal as it tries to resolve. If you choose to use both modes and both targets are illegal at that time, Winterflame won’t resolve.