I took the USMLE 10 days after the COMLEX last year and I felt it was a good amount of time in between, although I think a week in between is enough. You need to give yourself enough time to:
1. take at least one day off after COMLEX-dont disregard how burned out you are going to be after that test. It is 2 days of sheer hell and you will barely remember your name when you walk out after the second day. There is no way you will be able to study the next day let alone take another test
2. There is a lot of molecular biology and biochem on the USMLE that is not on COMLEX and you need to give yourself enough time to brush up on that stuff. I would suggest trying to read first aid at least one more time, do questions and do biochem/molecular in between the two exams
Good Luck-I know it sucks to take both but I'm really glad I did it!