The best split leg running shorts were back in the early 1990's when Nike had a line of "Internationalist" Race Shorts that were fully split and the inseam was about half an inch. The material was cut so that the side seam was about 4 inches and split. They were awesome shorts. I had three or four pair and loved them. Today, full split shorts are really hard to find with only a few brands still realy selling them. BOA has a great 1 inch inseam full split short and is my current mainstay for running (unless I am at the beach, in which case I wear a brief swimsuit).
http://www.boausa.com/