the drive letters are always going to change based on which OS is booted.
Because the simple rule is that the active OS drive will always be C
Frankly I had a functioning dual-boot setup between Win7Pro and VistaHome-prem both 64bit.
The "issue" I had with it was that it would always hang on a black screen and ask which OS I wanted booted (no time-out) and dismounting one drive made BOTH non-functional
I then went back to the way Ive always done multi-OS computer setups.
Seperate drives in removeable drawers but only ONE drawer mounting tray in the computer case so only one OS drive would be physically mounted at any given time.
Yes this involved shutting down swapping the drives and restarting but that only added 10-15seconds over a simple reboot, but without the inconvenience of having only one bootloader serving multiple OS installations.