Can I blend two animations with specifing bones that will be animated?

Hello,

I have two different animations. At one of these animations character sits and does nothing else. At other animation character shakes his hands while he stands.

Can I mix these animations some how, character both shake his hands and sits at the same time. Or should I create a different animation for that purpose?

Edit:
With theANMATOR2b suggestion I found solution.

I firstly created mask avatar with cheking all bones except hands. Then I created a new layer with adding this mask to that layer. Then I created same animation state that I want to blend with same conditions. And finally I adjust weight (from gear button next to layer label) to 1.

Look into avatar mask and animation layers. This will give you the information you are looking for.