If I understand correctly, your goal is to route traffic from IP1 and IP2 through different SIM cards.
The issue is that RUTX50 has a single modem, meaning that it can only be connected to a single carrier at any time instance. Switching between SIMs takes time, as it requires to terminate one connection and establish another, though it is not a suitable solution for real-time applications.
Your desired configuration could be achieved using RUTX12. This router has two LTE modules, and splitting the traffic between different mobile interfaces can be achieved following these instructions.
With RUTX50, however, this configuration is not feasible, unless RUTX50 has an alternative WAN source, wither wired or wireless.