You cant have 2 Sims active at the same time, but what i think you're after is a failover connection. ie when one carrier looses signal, it could switch sims and enable the 2nd sim service (different carrier prefered) with a short downtime.
This is doable with any dual sim RUT.
Load Balancing is when you want to Balance/Share the load of multiple devices over multiple WAN connections. Not usually used for a fail over type scenario, though it can be.