The Microsoft rule is that the older operating system has to be loaded first. If you try adding NT to XP, you're likely to get a blocking message preventing it.
The only way that I know of to cheat this effectively is to use Partition Magic run from it's rescue floppies to set up a second primary partition for the NT, setting it then as the active partition. PM will accept that even though there's no system in there yet, so if you load the NT as you reboot after removing the PM floppy, the installation should proceed OK.
However, this will not provide a boot menu for you, so you'll either have to edit Boot.ini to provide one, or use the Boot Magic provided with Partition Magic if it's a latest version that supports the NT systems. Boot Magic with PM 6 and before won't support XP etc, since the older versions had to see a DOS partition or system.