You either have Win 7 - 32 bit or Win7 -64 bit. There is no "booting into one version or the other". Retail versions of the OS come with both. The easiest way to instal both would be to use separate HD's and put one version on each HD one at a time with only say the 1st drive connected for 32 bit and only 2nd drive connected for 64 bit. Then you can connect both and use the BIOS boot order to slct one or the other.
With the Professional and Ultimate versions you can "Run many Windows XP productivity programs in Windows XP Mode." But, that is the closest you are going to get to 32-bit from a 64-bit kernel. Any specific reason why you want 32-bit mode?