One reason that we haven't been playing LNOE for quite some time now is because my nephew isn't very good at playing the Zombies, and my girlfriend always prefers to be teamed up with me instead of playing against me. So by default, when the three of us play LNOE, my nephew uses the Zombies. And then I realized just now, maybe we can switch that around? Two zombie players (me and my girlfriend) against my nephew.
Has anyone ever tried this before?
Personally I don't like playing with 2 zombie players, I feel like it is easier to play the horde as one mentality, hive mind, if you will. That being said, I'm sure it would work fine but micro managing the four hero characters may also prove difficult for your nephew. Sounds like you need to find a partner for your nephew!
Don't see why it wouldn't work as long as your nephew is capable of managing the team of heroes. We have played a few games now with two zombie players. It works out very well and it may even be a little tougher on the heroes.
While playing with 2 Zombie players would require some synergy between the two (thus, seem to be harder than with just one player), there's actually one aspect where playing with 2 Zombie players is easier - when spawning new Zombies. As per the LNOE rulebook, page 8, the two Zombie players are only limited via their color of Zombie, it even says that the two players can double up on the same Spawning pit. Very handy especially if a new Spawning pit appears at a strategic location.
But both of you are right - when playing LNOE or IFOS, my nephew has a harder time managing 4 Heroes than managing all of the Zombies/Martians.