I'll try to explain it a little better.
The SATA drive doesn't have master/slave settings, so you don't have to worry about that.
But make sure your old drive is the first in BIOS hard disk boot order.
Start your old XP installation (the 32-bit).
When XP is up and running - insert the 64-bit XP disc. It should auto-start the installation process.
During setup - choose "New installation" - later on you will be asked where you want to install XP 64-bit (the new HDD, of course).
A dual boot configuration will be created - when you start your computer it will ask you which OS you want to run.