On our devices, mob1s1a1 and mob1s2a1 are just logical interface names for SIM 1 and SIM 2. The actual Linux system sees them both as one physical interface - wwan0. mob1s1a1 and mob1s2a1 were simply created for the management of both SIMs to be easier.
While you are correct that these interfaces do not work like the others in the Failover section, that is because SIM Switch feature is used for switching between the SIMs.
Bonding is yet another topic. Your understanding of bonding is correct, it "merges" the data stream from both SIMs into one, thous increasing the throughput for a single connection. In our routers, we only offer Load Balancing, which can increase the throughput, but multiple clients are needed to utilize it (a single client will not notice the increase). More information can be found here. We also support bonding using a third-party package Bondix.
However, as RUT955 only has one modem, you will not be able to utilize bonding or load balancing between the two SIMs, it's only possible to perform it between a few different sources, that can both be active at the same time (both SIMs cannot be active). If you'd like to combine the throughput of 2 SIM cards, I'd recommend looking into RUTX12, which is currently the only device in our lineup that has 2 modems.