Hello John,
the short answer is: SIM card slot bonding is not possible. By default, slot 0 is the primary, and slot 1 the backup.
You can have a look at the link below for details:
Configuration of Dual SIM Feature and SIM Failover
http://www.cisco.com/c/en/us/support/docs/interfaces-modules/lte-wireless-wan-interfaces/200455-Configuration-of-Dual-SIM-Feature-and-SI.html