Howard,
Easy way is to use the OBitalk Portal, it basically automates/rewrites your call routes. Can also be setup manually in the OBi unit itself.
If you use the OBitalk portal you can include both units in the circle of trust of each individual unit. This will automate the change in the inbound call routes where any unit calling the other unit will go to the AA. This can also be accomplished manually by typing in the following. Note that you can also provide the AA to family/friends who do not own an OBi by putting their tel number in the inbound call route and sending them to the AA first.
Setup Wizard>Inbound Settings
ITSP InboundCallRoute
{(5145551111|5145552222|:aa},{ph} <- tel number you want to include in your circle of trust
OBiTALK InboundCallRoute
{(290111111):aa},{ph} <-- each OBi will have the OBi number of the other unit.
POTS-line InboundCallRoute
{(5145551111|5145552222|:aa},{ph} <- tel number you want to include in your circle of trust