Hi Andrew,
I would lean towards using a single 20G with both ports connected. Even with the slightly higher gain of an OMNI-400, you’re not going to get as much carrier aggregation - when the two “channels” in the LTE radio are bonded together to two different or similar nodes on the remote tower. In a lot of low signal areas, I see this providing a better overall connection and usable signal than a single channel.
Hope this helps!