But they're both the same version of Windows, right? ie you paid for Ultimate, so you got an Ultimate 32-bit disc and an Ultimate 64-bit disc, right? Microsoft allows you to use that key that you got for either version, but not at the same time. Each key is for licensed for 1 copy of Windows, which means you'll have to buy another key.
What version of Windows did you buy though? You can use XP Mode if you're on Professional or Ultimate, or you can just use your own copy of Windows XP with VirtualBox or Virtual PC (both freeware).