For Windows XP one version has to be installed after the other while both drives are plugged to see the first added into the master boot record and boot.ini file. The second Windows then becomes the defaulr version with the option seen at startup for choosing the other one. In the bios you would set the default version's drive as the first unless the entry is set for the first version's drive.
If you installed each separately with the other drive unplugged you would have to reinstall or repair install one to see the other added in. First see if the 64bit version has a boot.ini file identical to one seen in the 32bit versions. You have to uncheck the "hide protected operating system files" option seen in "tools>folder options" iin an explorer window. Someone who has run or is running the 64bit version can add a little more on this.