Why are there two LTE antennas?
Well, first thing to understand is that what you're asking is basically a content of a couple of university courses, so it's a bit difficult to give an in-depth explanation. 

In short, you didn't give a link to modem, so we can't be precise, but there are some general statements that can be made:


Should be.
Modem will combine both antennas.
It's complicated!


In short, category 4 LTE is trying to do so-called spatial multiplexing. The idea is that the antennas will be sufficiently distant, so that (oversimplifying here) each antenna can send its own independent data stream. To do that, you need sufficient number of antennas (also called "layers") at both the transmitter and the receiver. If you disconnect one of the antennas on your modem, you've just completely lost a spatial stream.

Cat 4 devices have one uplink spatial stream and two downlink spatial streams. So, if you remove a spatial stream, then you're more or less down to category 1.

I'm also not sure if the modem will try to do anything smart when transmitting, such as trying out which antenna is better for transmission or not. If I remember correctly, in category 4, there shouldn't be any uplink beamforming or anything else that would require more than 1 transmit antenna, but it's been a while.