I think what you need to do is reinstall XP on the secondary drive WITHOUT the primary drive being present - disconnect it physically. Otherwise the new install sometimes has the tendency of placing the secondary's drive's boot files on the primary drive.
Then finally reconnect the primary drive, setting this as the 1st boot drive in the BIOS, & see how it goes. If it doesn't work as before (i.e. 'at boot up both choices were displayed but automatic start up was to the primary drive'), you may need to edit the boot.ini file on the primary drive so that it does. We'll tackle this if & when it doesn't work as before. G