I’m working on evolve based game, so i need to change different body parts in runtime. Where was no problem when i use epic skeleton for modified arm (like swap hand for a blade), but i don’t know what to do if i want to replace arm with, for example, whip or something with 2 elbows, ets.
Also I want player to be able to replace lower body with different ones, like snake or spider lower body.
So I want to combine 2 (or more) different skeletons and skeletals meshes in one character.
I don’t think you can manipulate skeletons during runtime. What you can do is moving different meshes together to make it look like they form a single body, and spawn / despawn them to your liking.
So if i need to make something like naga/arahna i need to make lower body as separate actor, make a socket in it and load upper body into that socket.
And for animations i need to sync all body parts?
Here’s a tutorial for you 
You would need to work on some extra stuff though since you stated that the parts that are gonna go in the same place will have different skeletons and not just different models. Also, this video doesn’t demonstrate the switching mechanic you want. There’s more research and experiments to do on your part.
Check Cascadeur, it has some good animation features that use ai and their could be a mesh repalcement parts that fits with your problem. Good luck with the game