I assume it opens the 2nd one read-only (so, you could do a Save-As and merge your edits at a later time)?
Actually no. It does some sort of sharing thing. Online edits have to be "started" for a given rung, so that rung is locked for others while this occurs. It is more tedious to edit online, but very hard to do anything unintentional and very easy to undo those edits after testing but before "compiling" so-to-speak.
I don't know all the details as I have only been in that situation a few times. But I can easily monitor anything from one computer while others make changes from another. You get a pop-up informing you changes have been made by others.
I'm not requesting anything different at this point, though even read only would be useful. Having data view edits and forcing could be really handy though!