Hi E.,
We have twins also, only 9months old, and they play rough sometimes, too. I haven't had to time-out because of it yet, but I do separate them (mostly to give the one on the bottom a break!).
I have a couple of thoughts re: your post...first, I think it's probably more like play to them than anything done out of aggressiveness. And, I think separating them is your best bet. I think spanking just teaches them it's ok to hit when you're mad. I know it must be frustrating, especially when they're laughing and doing it, but this just makes me think they're really playing.
And you're right, keeping control of your voice and actions are important. Can you just say "no" or "we don't hit", separate them, and model nice play? Like show the offender how nice it is to be nice to the other one? Put positive actions and energy and reward to the behavior you'd like to see?
And don't underestimate the power of a well-placed time-out. My willful, bold and beautiful daughter had her first time-out when she was like 7 months old! She shouted at me with every little bit of anger her wee body could muster because she didn't care for the veggies I was offering, so I turned her around in her highchair for 30 seconds to let her cool off. She was so surprised that no one was paying her any mind that she changed her tune, and when we turned back around, ate her squash and peas. I think they do "get" a time-out at this age...they can at least realize that it's a break in the action.
I think right now you're modeling hitting, with a tap here and there, so I would eliminate it altogether. I would say a firm "no.", separate them, play and praise and really pay positive attention to the one who didn't hit, as well as try to catch them while they are playing nicely and gush about how great it is.
Hope this helps!, A.