If you have two spells that are the exact same lets say for example two frost spells and you cast them can you just cast two spells or are you forced to combine them?

You can cast them separately.

They only combine if you click the Triggers at the same time.

If you press L, you will fire your left hand spell. If you press R, you will fire your right hand spell. If you press them both together, you will fire them combined.

You can fire them individually, as long as you press the buttons separately. There is no way to cast both separately at the exact same instant.