I don't think you can go wrong taking one earlier than the other, just personal preference i suppose.
I studied as if I were taking USMLE (less focused on molecular genetics and biochem) and spent a day studying OMM prior to COMLEX. Used First Aid and QBank for USMLE.
Took a week off to brush up on molecular, biochem, and review one more time and took USMLE.
I thought COMLEX was easier and since DO students do better on that anyway, I would suggest taking that first and getting a feel for taking the board exams and the computer program.
If you take USMLE, you should aim to do equal or better than your COMLEX (or else it can hurt you...as in, you did well on a DO exam but worse on a MD exam). So using COMLEX to prep for USMLE was my approach.
Warning though, your friends only taking COMLEX will be done, you'll go out and party, and thinking of studying for another week is torture. It's questionable how much you really retain in addition to what you already knew in that time between exams. I think 2 - 6 days between the exams would work.
Just what I did and what I'd do again. Good luck! It'll be over soon.
CP