As I understand it, there is just the one modem so it can only use one SIM at a time hence load balancing between the 2 internal SIMS is not possible.
However there appears to be the option to load balance between 2 active WAN connections, so you could in theory, plug a modem with a 2nd SIM into the RJ45 WAN port and load balance between that and one of the internal SIMS.
I've not tried it myself yet but intend getting around to it at some point.