Yes, the two series crossed over quite a few times. Usagi was featured in the TMNT comic, animated series, and even had two toys in the TMNT line. And, unless I'm mistaken, I even read a little short comic in Usagi where Leonardo teamed up with Usagi. I actually got a bunch of the Usagi collections at one of my local library, and I never watched the TMNT show as a kid, so I didn't even realize they crossed over until I saw that cross over comic with Leo and Usagi. After that I read up on it and realized he had two toys and everything (I even own one now, I found it at a yard sale once).