[RIIM] Can RIIM be operated with two or more gateways in a single network?

Yes, you can implement a backup border router. For example, you can have your normal network running on PANID ABC1. On the side, you can have a border router running on PANID ABC2, just waiting there with no connections. Then, you create an event-handler stating that if devices on the network ABC1 lose connectivity for a while, then they switch their PANID to ABC2. The network will then connect to PAND ID ABC2 automatically.