Just forward the call to Phone2 when Phone1 is busy. You also need to disable Call Waiting, otherwise the OBi won't recognize a busy. Use OBi Expert to make the following changes:
Voice Services -> SP1 Service
CallForwardOnBusyEnable: Checked
CallForwardOnBusyNumber: ph2
Physical Interfaces -> Phone1 Port
CallWaitingEnable: Unchecked
The same forwarding parms are found on the Phone Port. You might want to make the changes there, depending on how you want to handle the forwarding.