I'm very new to all this as well, just being diagnosed about a month ago, so maybe some of the experts can explain it better, but I'll tell you what I know. My G.I. was leaning towards the Chron's diagnosis due to symptoms that I had outside the colon (mouth ulcers) but the colonoscopy/biopsies showed severe ulcerative colitis. During a conversation he would refer to my condition as ulcerative colitis, so I asked him which I had, and he said there was a kind of overlap between the two, that I had Chron's-colitis. He also said the treatments were the same for both. I'm currently on Colazal and Prednisone for 2 months before I return for a follow up appt. So far after 2 weeks I've seen no improvement. If the 2 months pass with still no improvement I'm seeking a second opinion from a different G.I.
Technically the difference between the two: Chron's can affect the entire GI tract, from mouth to anus. The ulcers that form in the colon can go deep into the tissue, and fistulas can develop. Ulcerative colitis only affects the colon, and the ulcers only affect the innermost lining, not going as deep as Chron's. As far as how these diseases can overlap, I'm not sure. I understand that in the beginning it can be more difficult to discern between the two, hence the split diagnosis, but as time goes by the diagnosis can become more definite.
Hopefully someone can add to this because I'm still trying to make sense of it myself.