I have two copies of a strategy card in play that lets me take an action, when my opponent declares an action (surprise assault, I think). Can I play both at once? ie. My opponent declares an action, then I play both cards, activating two units? Or does my opponent declare an action, I play one card, then he takes his action, then he declares his next action, then I use my second card....

You can play both cards...

Just before he takes his Action, you interrupt him and play the first. Then you excecute whatever action you wanted. Just before your opponent takes his action, you again interrupt him, and perform your action. Now it becomes his turn.