If you are getting value and enjoying yourself with both, then feel free, because adding in other content doesn't hurt. The big question is "are you enjoying it?" Try both methods and see how things go. If 30 minutes of FF + 30 minutes of Rosetta Stone (or anything else!) works for you, then by all means.
Try both methods. Usually you will find that one of them is more enjoyable, and that is the one you should stick with.
