I've found a slightly insane workaround to merge/combine two or more skeletons along with their animations within a Spine2D project. It works for Spine 4.2, and perhaps earlier versions. Sadly Spine doesn't offer an easy way to merge skeletons, so this is the method:

Step 1. Duplicate the skeleton you wish to copy (skeleton B) and place everything inside a container bone. Make sure the name of the bone and other bones don't coincide with those in Skeleton A.
Step 2. Duplicate the existing animation in Skeleton A as backup.
Step 3. Drag/drop the container bone from Skeleton B2 (the copy of skeleton B) to Skeleton A. This will move the rigged skeleton without the animation. This might break the Draw order, so adjust it accordingly.
Step 4. Select "Import project" and choose the same project you're currently working on. Be sure to check "Animation", and not "Skeleton" on the Import dialog. Click "Import" and on the next dialog, select the skeleton you wish to copy the animation from (Skeleton B in this case), and the destination skeleton (Skeleton A). Click on the animation you wish to copy. Do not check "replace existing animations".
Step 5. Now in the same skeleton you'll have the original Skeleton A animation, and the Skeleton B animation seperate from the original one. If you wish to combine the two in a single animation, continue to step 6:
Step 6: Without selecting anything in the scene, select the newly pasted animation and copy all the keyframes. Then switch back to the first animation (the original Skeleton A animation) and paste them on frame 0. This will combine both animations into a single animation within a single skeleton. Another way to combine the two animations is to manually select all the bones, slots and attachments, and copy them that way, but this is more tedious.

I hope this was useful at least to someone who's stuck thinking they have to remake a whole animation timeline from scratch. Good luck!