Yeah, this is easy to do using Shared Projects:
- Add YourGame.SharedProject to the solution
- Add YourGame.DirectX to the solution
- Add YourGame.OpenGL to the solution
- Add references to the shared proj to the other two
- Move all your game code to the shared proj
- Moving content is a little trickier: Add the .mgcb to the shared proj, and then open YourGame.SharedProject.projitems in a text editor. Change the xml tag of that file to MonoGameContentReference
So now you should have the shared project with all the logic of your game, and the other two are literally empty with only the Program.cs file. I use this technique all my monogame projects, there are a bunch of examples up on my github: https://github.com/dmanning23
Hope this helps. Cheers!