Master and Slave are remnants of the old IDE nomenclature; don't exist w/ SATA drives.
Over the last 20 years, I have either used a 3rd party boot manager or simply selected which HD to boot from via the BIOS. If its a new build w/ UFEI BIOS, it's especially easy .... simply go tot he Boot tab and select the normal boot or the "boot override" which the default will normally be the 2nd drive.
1, With both HD's installed, and puter shut down, unplug the data cable to the 2nd HD (connected to the SATA 2 port).
2. Boot and install Windows 7 (to HD connected to SATA 1 port).
3. Turn off machine, unplug data cable to SATA 1 port (1st drive), plug in cable to SATA 2 port (2nd drive).
4. Boot and install XP to 2nd drive (connected to SATA 2 port).
5. Turn off puter, reconnect data cable to HD 1 (SATA 1 port)
At this point you can boot to either drive (either OS) via the BIOS a/o worrying about fixing boot.ini files, running bat files or anything else. Each windows installation doesn't know the other is there.