A good way to train splits and oversplits without stressing the kness is to use a small wedge so you bend your knee off the tall end of the wedge. We don't allow oversplits until the gymnast is very close to flat so prevent pressure the kness, especially the younger ones.
I have spilts on all sides and left and straddles (we call them middles) spilts on double overspilts and overspilts (one folding mat is one overspilt). Right I have, but I don't like it and I'm not down on my overspilt with it.
I have my right leg down, my left oversplit on a 5 in mat, and my straddle is about 2 inches off the floor.
gym4life, the best stretches for splits are really just sitting in your splits a lot and really working on the stretches beforehand (I think almost everyone stretches their lunge and back over a straight leg before splitting, right?)