Thank you all for your replies.

I found out: My main problem is my lack of knowledge of the warlock class - I will have to do some research there. But at least one warlock seems mandatory to have a tank the priest can focus on healing - that keeps single target fights a little less cunfused i suppose.

The idea i had was to find as many matching functions between mage and warlock and use those during regular fights, e.g.

Key 1 = frostbolt, shadowbolt
Key 2 = arcane expl, the warlock pbae (hellfire??)

So the idea is to keep spamming the 1-key on one keyboard that controls my dds during a single target fight while being able to concentrate on my priest to heal or react to what ever needs being taken care of. The same goes with spamming the 2-key for pbae fights.

I realise that i will not use my toons to their best because I will have focus on a few abilities per class (during fights). From my experience on dualboxing wow, daoc and lotro I have found out, that my playstyle on multi-boxing is to concentrate 80% on one monitor, and 20% on the second (the 4 DDs) - I just can't handle more .

Anyways, any further input is welcome,
I am still debating (maybe 2 mages, 2 locks?)

The soon-to-be Bunnyz on EU-Durotan