That springs to mind is why? Do you have a cd supplied with your machine? If so why do you need 2 installations of the same os?
If you want to boot from the master as opposed to the slave.
1) Open the case
2) check to see if you have 1 or 2 IDE channels on your motherboard
3) check the jumpers on your hard drives
4) to boot from master as opposed to slave, swap jumpers, or ide cable as relevant.