This was a very exciting feature to get working for me.
Using several open source Java->Asterisk jars a very simple service is implemented through simple-methods that takes parameters to create a call to first the originator's channel and then to an extension.
parameters (* denotes required)
*originatingChannel - Will call this channel first, needs to be in the format "SIP/200" or "IAX2/200"
*extension - Will call this party after the originatingChannel has accepted the call
priority - not sure the actual usage of this one, if not passed into the service defaults to "1"
context - used with the asterisk dial plans, if not passed will take from trixbox.properties files
actionTimeout - timeout I believe for both parties to accept the call