There are 2 kinds of case.
Case A: Let say that you wanna the caller to press 1 to call a specific extension in another extension.
Step 1: Create an extension specially used for transferring the calls(e.g 4000 )
Step2: Then set up this extension to always transfer calls to an external number which is the extension extension number(e.g 5000) in another PBX. This extension should have the permission to call Branch PBX of course.
Step3: Set up the key press event of IVR, e.g, Press Key 1 to route the call to extension 4000.
Case B: Let say that when caller call PBX A, you want this caller able to call Branch PBX B's extensions as well.
Step 1: Connect 2 PBX via truanks. and set up a outbound route allow PBX A to call PBX B.
Step2: Go to IVR
Step3: Allow caller to dial outbound route, and choose the route which allow PBX A to call PBX B.
Hope above will help you.