it will give you a dual boot option as well when you install the XP drive again
No it won't, dual boot will not be created when the XP drive is added after the Windows 7 instalation.
It can be made to boot by using something like EasyBCD, but W7 will not create the dual boot option for XP when XP is added after W7 instalation. If Xp is already there with the drive connected, the W7 instalation will create a dual boot option. Remember he states he is going to disconnect the XP drive before installing W7, so how will it know to create a dual boot for a non existing OS.