You will have to play with this a bit to get it to work:
function YourWeaponImage::onReady(%this, %obj, %slot)
{
%obj.playthread(2,"armreadyboth");
}
Attached is a copy of my 'fireball beta' script that never got past development stage. It's got noticeable flaws, but it should help you out a bit. It raises both hands on a 'charge'. I'm not sure how you can make both of them have an attack animation, though.
Also, can someone link to the thread with all the playthreads listed? I can't find it.