I have got one part drawing, in which I created a part1 (one body)

I have got one exisiting, finished part (one body)

Now I like to create a multi body part containing the two body parts above with the complete feature tree.

I then want to edit (roll back and change) the existing part and after changing combine the two bodies in one solid.

How do I 'merge' the two part drawings into one multi-body part?

Thanks, Gerard