If I understand your question right, it sounds like you want to use Animation Tracks. If you move your "lip sync" portion of the animation into its own animation, so that the only thing in that animation is the lip syncing, and then your other base animation does everything except the lip syncing. Then you can use the track layering in order to get the effect you're after.
Specifically you'd set the base animation to say Track Index 0, and then set the lip sync animation to say Track 1 when you need both animations to play, and only set the base animation when you don't want the lip syncing to play.
Check out more information on this here: http://en.esotericsoftware.com/spine-applying-animations#Tracks
