I think this is likely with any pair of arts (Shorin-ryu Karate students who train with me have to learn a new version of zenkutsu dachi, which drives them a bit nuts). I wouldn't choose a second art based on whether it uses new stances or not - the stances are just support for the movement style and bases for the technique.One thing I'd consider when scoping out a striking style, be it Muay Thai, or something else, is stance compatibility. From what I've seen of Ninjutsu, they emphasize several rather unique stances, and another martial art may force you to operate from a different one. Just something to consider.