Until now I used Programming Lab. Its desktop can be opened several times on the same notebook. Now I try Wolfram|One with the standard Mathematica desktop. It cannot be opened twice on the same file. At least I cannot find this functionality.
Of course one needs several instances of larger files: Look at other functions defined elsewhere, copy/paste from elsewhere. Something like a multiple document interface ist needed. Or some kind of split-screen. Or at least a completely isolated window for the same file.
Using Mathematica...I think that .. if you open your first notebook and evaluate all values, then create a new notebook... you will have all values evaluated in this new notebook. So, you just continue working.
However, I dont know about saving both of them. Maybe you can use the newest and then copy the content in the main one. And, I also dont know if it works on Wolfram|One.
what you describe is not what I was talking about. I talk about using the same notebook in two windows.
I think I was wrong. I tried to open the same notebook again from within the notebooks File-menu. This doesn't work. But I can start a completely new session with the same notebook. They are unrelated, but I can use one for lookup and copy source and the other for paste.