Stacking frame animations from different files

Hello, I'm looking for a way to layer a frame animation from one file on top of a frame animation in another file. Each file has the same number of frames. Imagine one file with a frame animation of a moving background and another file with a frame animation of a walking person. I want to take the animated person and put them on the animated background, and have both animations going together.

My ultimate goal is to be able to automate the process because I need to do this to a ton of files.

So far what happens is I duplicate the layers from the vehicle file into the background file, but the vehicle frame data doesn't merge. So I have to reanimate it frame by frame.

The "make frames from layers" command in the timeline menu is great, but it doesn't seem to work if there are existing frames.

Fingers crossed over here. Thanks in advance.