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.