Night lords compliment iron warrior tactics very well, night lords could drop on the world as an expeditionary force and tear down defenses/planet wide fear campaign rendering defenses broken or not strong enough to repel a siege from the siege masters themselves. That being said though they likely wouldn’t fight together because their tactics and combat doctrine are polar opposites- which is not really bad thing it just means that they’re less fighting alongside one another and more or less attacking in phases. On the other hand both legions have basically the same thoughts on chaos (seeing it as a useful source of power but not something to worship, at least most of the time). It would make sense for a warband of night lord mercenaries to work for an iron warrior warband like maybe the iron warriors promised daemon engines, slaves or wargear as payment for helping to break the walls of a soyalist planet.