This can be done with any section of a model, but this tutorial will show how to have two different animations play for the upper and lower sections of the model.
First, open ModView and open any player model.
Expand Bones and keep expanding until you get to upper_lumbar.
Right click on upper_lumbar and click on "set as secondary Anim Start".
Next, go to the sequence that you want the upper part to have. When you find it, right click on it and click on "Start Secondary Multi-Locking with this sequence".
Then go to the animation you want the lower section to use, and right click and click on "Start Multi-Locking with this sequence".
Again, this will work for any bone set you choose to use the secondary anim start.