There is no master...when it comes to operating systems.
Since you have two computers, it seems to me that the simple solution might just be to network the two.
If you move a hard drive...from one system to another...with the intent of using the O/S on each...that may or may not be legal (as pointed out).
<<XP home is a legit version and XP Pro is a oem version.>>
I guess that I don't know what you intended to convey by that..."oem" means various things in the mind of users, most of which are not equal to the realization that...any OEM version of XP is tied to the original system it was installed on.
And...there are two concepts that come to mind for me:
a. OEM systems from manufacturers like Dell, Gateway, Compaq, etc. where the O/S is installed by the system manufacturer.
b. Microsoft OEM versions of XP, such as the ones which I have purchased. These are now known as "system builder" versions.
Either version is tied to the system originally installed on.
FWIW: Intel® Application Accelerator - Moving a Hard Drive to a New Motherboard - http://www.intel.com/support/chipsets/iaa/sb/cs-009290.htm
AND Moving XP to new MB or Computer - http://www.michaelstevenstech.com/moving_xp.html