Hi Gareth,
For outbound calling, this is done on the gateway using the Trunk Group table. Add all channels across both trunks into one trunk group on the gateway. In the IP to Tel Routing Table, direct all calls to that trunk group. When one trunk either becomes full or is not online, then the calls should failover to the other trunk.
For inbound calling, that is controlled by the telco, as the gateway has no bearing on which inbound calls arrive on which trunk.