RUT955 has one LTE module so you won't be able to activate both SIMS at the same time. However if I understand correctly, you want this solution for failover purposes.
You don't need to have both sims active for failover to work. You just need to set the priorities on failover screen. Simply press left mouse click on the interface, then drag sim1 to the 1st, sim2 to the 2nd and then press Save & Apply.
Let me know if that works for you.