that shouldnt really be a problem. means you want to run 18-light/6-dark or 20-light/4-dark.
it might take a few extra weeks, but I would suggest doing the reveg, until you can take a clone.
then cut down the reveg and grow the clone.
and for the future, try getting another box. it can be pretty small. and just run 1 or 2 cfls so you can take a clone, before you flower the plant. then the other plant should be ready before your other is done.