It doesn't matter whether they are being used "at the same time".  You are buying one "seat" that can be used on only one computer.  You can move it from one computer to another, but the previous installation has to be removed from the other computer.

Again - If you want to use MMM on two different computers, you have to purchase two individual "seats".  Otherwise, you are "pirating" the software, which is illegal and against the "End User's License Agreement" (EULA) that you agreed to.

MAGIX sells software.  It is NOT "freeware" or "shareware".