Hi,

> I'm sure there's more to it than specifying different ports to
> run on.

Well.. Yes, it is... at least I've had no problems doing that on Windows XP

What I've done on a new box was:

Install FB 2.1 (do not copy client dll to <system>, do not start)
Install FB 1.5 (do not copy client dll to <system>)
Change Fb 2.1 port on firebird.conf
Start Fb 2.1

(I also do not install Guardian nor any CP applet, but I think it does not
make any difference)

HTH

Regards,
Anderson Farias