My charname shaman has finished BGEE and is now in SoD up to recruiting M'Khiin. After seeing her summon goblin spirits in the cutscene, I decided to give her a go, and found a couple of rather odd side-effects between our two dances:
1. The spirit dance states seem to affect each other in strange ways. If M'k summons some spirits and then Charname goes into her dance, M'k is able to move around without her spirits disappearing. If we chain our dances, we can move all over while keeping the spirits alive, as long as one of us is dancing. Doesn't always work, and I don't know what exactly the conditions are, but it happens frequently.
2. The spirit count seems to be shared between us. If M'k has her spirits out, my shaman is unable to summon any more and vice versa.
3. And this one is by far the most important. Ever since I added M'k to my party, our maximum spirits was set to 2 - I was level 9, she was level 7, both of use should be able to summon 3 each yet only 2 spirits can be summoned between us both. Especially problematic, is that removing her from my party leaves me still only able to summon 2 spirits. Worse still, loading an old save (before recruiting her) doesn't correct it either, nor does quitting the game and restarting it. It seems she has overwritten a variable in the game somewhere.This appears to be solved, see comment below.
TL:DR - If you are using a shaman, do not recruit M'Khiin. Probably don't create 2 shamans either, but I haven't tried that.