First my question can't you just heal Astate? In my game she wasn't hit by Void dragon, only by the adds which did little damage. I didn't try if it was possible to heal her, but she wasn't in such great danger because she does have a lot of HP after all and adds don't do that much dmg.
If the healing doesn't work, why don't you just have try to have the damage directed at your characters or summons instead?
Well here are some tips based on my experience:
* If you take those adds down to 1-2, Void dragon will only summon new ones one at the time, so keep the adds in manageable numbers. So leave the "most harmless" adds alive and let them be alive. If you have a good tanking character that could work. --- In my game I had one add shooting some 50-90 damage bolts to Astate and my casters, but those were pretty harmless.
* Chain Crowd Control to Void Dragon worked for me most of the time. Especially freeze seemed to be effective, stun also worked sometimes.
* Lastly the obvious one: keep all your characters spread out so that once Breath hits, it only hits one character.
I imagine it's difficult with two characters, however I think you've got also more action points per character. It boils down to:
1) Good positioning, so you decide who takes the damage and all the shit that void dragon throws at you
2) Having enough abilities (heal, purification) to stay alive
3) Ability to deal some damage