You can assign up to 2 motions to an actor, to allow mixing the motions together. Usually this is used when you have one motion for the body and another for the facial moves.
If the 2 motion use the same bones, the second one will override the first. If that’s not what you wanted, try to change the order the motion and see if that helps.
To assign a seconary motion, open the motion menu, tick “Second Motion” and then make your selection.
To clear a secondary motion, simply untick the “Second Motion” box and select the main motion again.