I've never understood these concerns on a Mac. Macs are not Windows computers and that has never been a concern. Macs have always had multiple windows capabilities and cutting and pasting between windows with different apps has always been a breeze. You can have many different apps open and cut and paste between all of them regardless of what you are cutting and pasting. Apple has always handled multiple screens easily also.
I also have not seen a single problem with legacy software between El Capitan and older versions of software except for some vertical solution software which most people would never run into. At my office the makers of MacPractice would not let us upgrade to El Capitan for several months until all aspects of MacPractice were vetted with it. We finally updated in the first part of June when the 3D radiography software was finally checked off.
El Capitan is rock solid as far as we are concerned.
I admit I don’t really get the “side by side” thing. Could be I just don’t get it. But if you read the CNET stuff, they make is sound like it is something Apple has been playing catch-up with.
I guess it is just a workflow I have never had a need or desire for.