I would like to open a new MonoDevelop instance to work on a different project on the Mac, and the OS is currently preventing me from opening a new instance.
Just Open the Terminal write or below command , you can change instance name number e.g(/Xamarin\ Studio1) it's open another one Xamarin Studio , e.g(/Xamarin\ Studio1)
$ open -na /Applications/Xamarin\ Studio.app --args -DataPath /Users/$(whoami)/Library/Application\ Support/Xamarin\ Studio1
Using above you can create any of .app multiple instance .