This tip is for ant version 1.3 (haven't tried with older):
When using multiple VSS databases, the default database
can be set with environment variable: SSDIR

It must point to the folder where the database specific
srcsafe.ini file is located.
Example: database (srcsafe.ini) is located in folder:

G:ProjectsUltra

and other in folder:

G:InternalUtilities

When using the Ultra database, set SSDIR=G:ProjectsUltra
and when using Utilities project, set SSDIR=G:internalUtilities

This tip is for ant version 1.4:
Documentation says that serverpath attribute should
be used to indicate the location of srcsafe.ini
Example: With the information in previous example,
just use attribute:

serverpath="G:/Projects/Ultra"

or

serverpath="G:/Internal/Utilities"

With different targets.

I tried with both ways and they work just fine. The new
parameter in 1.4 version is of course a lot of simpler
to use. With environment variables it is always little more
work to set them correct when changing databases.

As a tip, create for each developer a custom property file
which is included in the build file. There they can set the
exact path of the srcsafe.ini folder (the drive mappings might
not be same between developers).
So the 1.4 example should look something like this:

serverpath="${VSS_ULTRA_PATH}"

or

serverpath="${VSS_UTILITIES_PATH}"

I advice you to use version 1.4 because it is much
simpler to use with multiple databases and it has
been released.