This is from my experience with the tutorial level.

1. Combat is chaotic. Let's say your character is in prone (spacebar) but you decided to take a combat stance (C), it will result to a crouch position which may hide the character (if behind some boxes for example). HOWEVER, if you switch to a second character, the character will immediately let go of the crouch position and will stand up, resulting in getting spotted, and chaos.
2. During combat, if you switch characters, they will most likely move to different positions, character will abandon cover resulting to chaos.
3. Plan (the game mechanic) doesnt work well if the intention is mixing it with combat. It cannot comprehend multiple actions from one character.

There are ways to mitigate problems with combat such as using only one character at a time and then proning them and switching to another when they reload. Use the crouch position to advantage. It works as some sort of prone position and can hide the character under certain circumstances.