*sigh*
Misinformation sucks.
You have a couple of options:
1. You can start from scratch and load XP first, then Windows 7.
2. You can disconnect the Win 7 drive, then repair Windows XP on the other drive, reconnect the Win 7 drive and use your BIOS boot menu to select which drive to boot from. (This is usually accessed by pressing F12 during boot... sometimes F10).
The file systems are interchangable... they are both NTFS. Windows 7 does have a newer version of NTFS, but that doesn't mean each OS can't read the other's files. The bootloader however, is different. Windows 7's bootloader will recognize XP, but XP's bootloader will not recognize Windows 7. (which is why you load XP first when dual-booting).
As for hiding the OSes from each other... I'm not sure that is possible. Both drives will be NTFS... and as long as both drives are connected, they will be visible.