It seems however impossible to have 7.5 and 2010 open at the same time…
It is indeed either-or. Either you run 2010, or you run 7.5. You cannot run both at the same time.
This is true even on the same computer. So while you can run multiples instances of 2010 on one computer, or multiple instances of 7.5 on one computer, you cannot have instances of both.
Each of them pulls a different license, so as soon as you start 2010, that is a one-seat license checked out. When you start more instances of 2010, they run off of that same one-seat license, so you are not exceeding your seat count.
But, when you try start 7.5, that tries to pull a second, different license. And that exceeds the seat count (assuming you have just one seat).