The Northern Kingdoms did fight Nilfgaard together at one point in the books. But also didn't. Also, wasn't a united Northern Kingdoms something Djikstra wanted to achieve in TW3?

And about Witcher neutrality. It doesn't really exist. It is just his excuse that makes the world think Witchers can be used as tools. They can instead choose sides themselves, whatever works best for them or people they care about (however ironic that last part sounds). Initially, Geralt thinks he is neutral, emotionless, not a worthy human being. But his character develops and later he realizes that it is impossible for him to remain neutral, and has to choose sides. This culminates in the last book. That's probably why he fails to remain neutral in the games. Most of the decisions you have to do are based on his(well, yours) feelings and morals, something he was convinced he doesn't possess (until the Shard of Ice story I think). I think the same applies to all Witchers, it's not just Geralt that's bad at this (Lambert+Keira for instance). Pretty useless discussion I guess, just felt like typing a bit :D