This happens a lot as narcissists choose narsissitic mates much of the time. They deal with each other the same way they deal with everyone else and they form a sick and twisted relationship that's bound to have its share of pain and suffering. The kids are at risk especially since they are cut off from any potential of having a good parent, unless the kids are taken from them.
Watching 2 narcissists fight could be very interesting, even ammusing if you realized what was going on. You could bet money on which one would outsmart or outmanipulate the other.
A narc cannot play another narc. It's like a user can't use a user. They know who they can suck dry and who they can't.