Yes, you can with LtAP or RBM33G using 2 LTE modems, I'm connected with 2 different ISP and load balancing connections. With torrent downloads both ISP are used simultaneously.
Sorry guys, indeed if you have a RB where you can install TWO LTE modems, then you can use two networks. I thought this would be clear to everybody!
I was referring to devices like LTAP Mini, which has 2 SIM card slots but only one modem, on these devices you only have one SIM card active...
LtAP mini and SXT I know that have 2 SIM ports but one modem only, anyway now you are right.
By the other hand is true that anyone needs to be careful with the number of SIM slots and the number of modems that you can install and
use simultaneously. For example, LtAP have 2 miniPCIe slots + 1 USB and 3 SIM slots, so, you can install 3 modems, but you can use only 2 simultaneously. That one device have X number of SIM slots (Except 1), not means that you will can use the same number of connections simultaneously. Always do check for hardware limitations using block diagram available for download.