I may have just gotten lucky, but when I did Split Personality, I went all-out DPS before the split. Once she split, I kept it very slow and methodical. I started with a 4x Chain Lightning, then switched targets, did one cast, switched again, did another, and just kept bouncing back and forth between targets, making sure I have the one with the highest health on target before casting. Then, once they are all down to about 20%-ish, just went all-out DPS again.

I was doing Nexus again yesterday to finish up Intense Cold (used the 'stand at the entrance' trick with my team) and I was able to beat her before she split for the 2nd time, so if you can manage to do that, it'll certainly help. When I got Split Personality she split twice on me.