I'm assuming you have two completely installations on separate HDD's, both with an MSR reserved partition, and both marked as "Active"? Of did you end up with a single MSR on one of the drives and not the other? Then you change the boot order with a hot key like F12 to the desired Win OS?
There is a difference in the boot blocks where the MSR on Win-8 takes 350MB as compared to 100MB for Win-7.
You might try placing the Win-8 drive on SATA_0, and the Win-7 on a higher number SATA_#, to see if that works.
They should be completely separate with their own boot blocks, so that if you unplugged the power to Win-8, Win-7 would boot properly even if not set first to boot.