Adabe,
Yes, you would need to purchase an addtional license.
The consumer EULA for XP allows one installation on one computer at any one given time. So a dual boot situation like yours would require a second license, regardless of the fact that you can only use on at a time.
To read your EULA, click Start>Run, type winver and hit Enter. Click the link in the resulting dialog box to be shown your EULA.
BTW, I was under the impression that XP would not install and run from an external hard disk, it needs to be on an internal disk (unless you are using eSATA).
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp