AnimationState.AddMixingTransform

The name is odd, because it does two things. the first time you use it, it limits the animation to only that bone chain (the bone you use, and kids.) The 2nd+ times you use it, on the same animation, it adds that bone chain. So, if you want both arms, AMixT(upperLarm) and AMixT(upperRarm) will do it.

To combine two animations, simplest way is to put them on different layers. The higher number wins, but it you split into upper/lower body, that won't matter (you get first pick of the pizza, but hate crusts, and I only eat crusts.) Looks like:

 animation["topRun"].AddMixingTransform("upperSpine");
 animation["topAim"].AddMixingTransform("upperSpine");
 // both are on layer 0
 animation["bottomRun"].AddMixingTransform("lowerSpine");
 animation["bottomRun"].layer = 5; // any > 0
 animation["bottomCrouch"].AddMixingTransform("lowerSpine");
 animation["bottomCrouch"].layer = 5; // any > 0

 if( something) animation.CrossFade("bottomCrouch");
 // stops other bottom (layer5) animation, leaves top alone, play crouch

 if( something) animation.CrossFade("topAim");
 // stops other top (layer0) animation, leaves bottom alone, play aim