How about sending callers from IVR on the first pbx to a dedicated ivr on pbx 2 that is only for dialing extensions?
Have your IVR on PBX#1 say "If you know the number you wish to call, please press 9 now"
Have #9 go to an IVR on the second PBX.
The IVR announcement simply be "Please dial the extension number of the person you wish to speak with".