Desktop app: Opening/editing multiple notes

It’s unlikely that notes side by side will be supported (maybe tabs at some point but not all opened at the same time). A possible alternative, depending on your workflow, is to open some notes in an external editors, and keep the one you want to change open in Joplin, then you can copy text more easily.

A planned feature is to allow opening multiple notes in an external editor, so maybe that would support your particular use case.