The shingeki girls are admittedly unfinished, but I'll throw out my plan for it. There are 4 separate girls, and you can pick a minimum of 2. The two you pick will have a separate set of stats and abilities, and your condition is to make either of them cum twice. Sounds simple, but like in real life, if you try to have a threesome and only focus on one girl, you're going to have a bad time. You might be having a good time pleasuring Mikasa, but it may turn out that Annie has the overpowering trait which she promptly uses to force you into having sex with her. Now you're generating PP towards Annie, whereas you meant to focus on Mikasa.
So, basically, it just has more tricks up its sleeve. I haven't begun to balance it yet, but you likely need to have a few traits up your sleeve. The rewards will likely scale depending on the amount of girls you try to tackle. Depending on how this game as a whole plays out, you might even be able to bring a friend into the encounter.