Yes.

On Windows XP SP2:

Start -> My Computer -> Control Panel -> Network Connections -> Wireless Network Connection -> Change the order of preferred networks -> "Add..."

Add both the ESSIDs to that list. When it cannot connect to one, it'll connect to the other (actually, it depends on the signal strength of each one and the position in the list, but that appears to be unimportant for your needs).

Cheers,

-jk