I don't think it's possible to open 2 or more notes side by side. Like when you are summarizing a set of meeting or research notes... you need to scan through and cut and paste or summarize points.
In Evernote I used to open the note in it's own window and I can comfortably fit 3 on my ultra wide monitor.
The closest thing I could do is open in an external editor but the formating and layout isn't the same.
That is a work around but doesn't address the workflow... as I am scanning both docs section by section to see if I'm missing anything and need to add a detail. So really want the 2 or 3 docs side by side. I'll use the external editor for now but the formatting the throwing me off. Is there a recommended external editor that works well? and is free?
I use Sublime Text as an external editor on Mac. Decent looking markdown and opens super quickly. I do hope that someday Joplin will allow side-by-side, but for now it works.