Why? Don't know of any but... One only really needs Win7 Pro or higher & with Windows VPC & XP Mode, XP can be run on Windows 7.
Dunno if this (reply) helps... I may be completely missing what you are wanting to accomplish or why.
If you are meaning, have a dual-boot configuration (in a computer) of XP & Win7 on (removable) media... as I said do not know of anything that does that although, that certainly doesn't mean it doesn't exist. Even taking an 'image', is of a specific OS, in a machine but, not both & thier Boot Manager.
Just like Drew said,its virtually impossible to have 2 Os's on a removable media such as a USB or a DVD,it would be just like forgive me here [edited unacceptable expression] ,The OS needs 1 complete Media to store its information on.So One word-IMPOSSIBLE!!!