Multiple instances, one database?

[Gerald V. Livingston II]

OK, next question as I test this out. 

We currently have two companies in one building. Due some legal issues we
can't fully merge them and absorb the customer base of the second into the
first yet, we have to treat them as completely separate even though the
owner/employees/physical address are the same.

Can one copy of freeside be set up to create different invoices and use
different Auth.Net settings based on a domain name suffix on usernames or
some other flag?

Ie. joe at abc.tld should get all invoice or nitification emails with a
support at abc.tld return address and abc.tld template headers .. but
joe at xyz.tld should get messages from support at xyz.tld

If not, can I run two instances of Freeside on a single machine using a
single database with usernames stored with a domain name suffix (there are
some duplicates except for the domain name part)?

We are trying to merge billing into a single system in preparation for
future total merge of the companies.

Thanks,

Gerald