How would you go about linking to projects together, and adding a menu?

Unfortunately, There's no real way to have a Solution of Unity Projects.

Instead, you have two options:

1. combine both projects into a single project and create a Menu scene that selects a different game scene depending on your choice.

2. or Create a menu project which runs a different executable/build for each game depending on your choice on the menu.

Choice number 1 is the preferred option since you don't create a medium between your games and the mechanic of starting games.