I found a much easier way to load up two instances of sip communicator with different config files (and therefore different sip accounts).
It turns out that the first place that sip communicator looks for a configuration file is the current directory, IE the directory from which ant is called. So I just have a separate sip-communicator.xml file in my sc-avmailbox directory.
I'm not going to mention the system that I had in place earlier. Such Kludges ought not be discussed in mixed company.