merging is not possible, however there may be a way to get it to work though im not sure id recommend it. basically you could try making the second character a child of the first. this is done in the class settings if memory serves. there will be a option that says parent class and you just change that.
personally i would try to make the changes to the first character. from what you mentioned it seems like its mainly things associated with the skeletal mesh that need to be changed in which case it isnt too difficult. just open the first character and select the mesh i nthe components tab, then in the details panel change the mesh and the anim bp to the new one. there may be other options that need changing as well but thats really all i can tell you based on the information provided.