Yes I've witnessed myself and my ex partner doing it many times. I don't know if its the case in all instances, but when the need arose to go into damage control when either of us had 'gone too far' the NPD would mirror the BPD traits he either already had stored in a layer underneath or had picked up from me to calm me down because seeing him acting like me worked a treat LOL. When I'd done something really bad I would go all NPD. When I did something really, really, really bad I'd bring out every PD in the book and then some. Then he started copying and I thought this is really stupid. It's not a relationship it's a competition to see who has the most mental health issues and can get away with it! So I ended it in a very polite calm way with no PD games and he reciprocated.