The AppleShare server is designed to limit access by individual, not by
geographical location. So it's not possible, for example, for an individual
user to have differing access privileges depending on which workstation
he's using at the moment.
Once networks are connected, it's not possible through AppleShare or the
Chooser to limit inter-network access. Unless you're using a mail system or
other system communication that requires inter-network access, it may be
best not to connect your networks.