Dual Cell Configuration for M900

We’re trying to set up two M900 base stations with two M70 handsets for use by a cable car. One M900 is installed at the valley station, and the other is at the mountain station. The two basestation are in the same network - the valley and the mountain station is connected via Fibre Optic Cables.

Our goal is to have the two M70 handsets work seamlessly in both locations. Specifically, we want to be able to make calls between the handsets when:

• Both handsets are located in the mountain station.
• Both handsets are located in the valley station.
• One handset is at the mountain and the other at the valley station. 

We tried to setup this installtion with both base stations configured as primary stations. But it did not work - the two phones connected ether to the base station in the mountain or in the valley. So we can not use the phones ether at the mountain or in the valley. 

Can you give us some advice - how we need to configure the two base stations?