I worked on a project where we had two SSDT projects with different versions of SQL, one 2012 and one 2008. The 2012 project referenced the 2008 project as a “this database” reference and while this seemed like it worked, the reference in solution explorer had a little warning sign and it caused nothing but problems.
The first thing was that the reference didn't work on anyone else's machine when they checked out the solution, the other developers had to delete and re-add the reference once.