I don't see an issue with reuse. I would definitely let it dry out some, or thoroughly mix it with some new soil. Considering how inexpensive potting soil is though, can't hurt to start fresh. Also might wanna mix in about 50% perlite to help with drainage so you won't run into that problem again. Also, if they're that old, you might want to consider putting them in separate pots. They'll be happier that way.
