I think the problem is that I was too narrow in my definition of what I considered to be skills.
I was focused on all the stuff I have to do solo and viewing my shaman group through those eyes.
I ended up thinking like a non-multiboxer.

Removing those blinkers, I can see that it's more a case of completely different skillsets.

With multiboxing, you are looking at the big picture, rather than focusing on one toon. You have to monitor the health of all your players, their position. I'm not just dpsing I'm looking at healing the group, other players around me.

And of course - even though I'm not sure it's a skill as such, it's the ability of being able to affect the course of the battleground. Sure you might not win it, but you have enough of a group to go off and target key tactical points.