Its a strange one but lucky for you I happen to have a customer with a few different Dymos.
I've set theirs up on the server and they have the same problem. Although the driver is similar, it doesnt work if both are on.
Trouble is, server 2003 is not officially supported as a print server for Dymos (server 2000 is though???)
Check the serial number on your 400 dymo. If it is lower than xxxxx-2096232 it isnt supported for print servers at all!
Sorry mate, this looks like a painful one.
Silly suggestion, but try setting them both up on an XP workstaion and sharing them from there and see if it works. Dymo suggest they do support XP print servers but dont specify if different types of dymo can be used at once.