How to get partial animations with mecanim

I have some animations that control one arm, and others that control both. (My animation just has keys on the right arm for example) I want to crossfade from one of the animations that has both arms to an animation that uses just one arm. The animations play on an override layer. I can create a mask on the animation (import setttings) that uses just one arm, but then the other arm is going to the relaxed pose. If I don't apply the mask then mecanim automatically creates keys on the first and last frame of the animation?

I would expect the unanimated arm to have the motion from the base layer but this can only be achieved by applying the mask on the override layer rather than the animation. Is creating multiple synced layers the only way I can achieve a good crossfade between partial animations? (additive animations are not an option for me)