I'd like to have 2 windows open to copy files from one solution to another - can I open 2 windows at the same time with different solutions? I can't seem to find the option to do so.
Yes it's possible.
From the terminal, using:
open -n /Applications/Visual\ Studio.app
For example, in Xamarin you're also able to build and run different solutions on different platforms (Android/iOS) using simulators.