A splitter is not going to change resolution, it will only report the commonly supported formats. It's entirely possible a Dish receiver may force a 1080i output if set to do so, the splitter passes the signal. If one of the TVs is 720P only, but can down-convert, you still have a functioning image. But if it can down convert, the TV may have reported 1080i compatibility to begin with.
I can't say first hand if you can force the receiver to output with regards to 720p/1080i as I don't have any 720p TVs. I do know 3D will not function if both TVs are not 3D.