technical details on external antennas

I will be using my AC763S in a remote location that requires an external antenna to get a signal.  I know the exact towers I will be connecting to, and I know the exact frequencies (LTE band 1 and/or LTE band 4) I will be using.  Commercial antennas tend to try to be all things to all people, and as a result are not always that good.  I can design and build antennas to fit what-ever frequency I wish.  So, I have some questions about the external antenna connections on the AC763S.

First, LTE FDD technology uses two separate frequencies for the connection, one for transmit and one for receive.  The separation of these two frequency can be significant (1740MHx v.s. 2140MHz), making it difficult to design an antenna that responds well to both frequencies.  Far better to design two antennas, one for each frequency.  On the other hand, the AC763S has two antenna connections.  How does it use these two antenna connections?  What are the detailed specs for what antenna's it expects to be connected to these antennas?  Is it reasonable, or does it make sense, to connect two antennas, tuned to two different frequencies (transmit frequency and receive frequency, AKA upload and download channels, althought this isn't really correct)?  Or do the two antennas need to be tuned to the same frequencies for MIMO technology?

I have checked all of these links, but none of them really gives the answers (the second, for example, tells one only the absolutely obvious - why do people waste their time on such useless fluff???):

http://kb.netgear.com/app/answers/detail/a_id/23901

http://kb.netgear.com/app/answers/detail/a_id/23385/~/how-is-the-external-antenna-connected-on-the-aircard%C2%AE-76xs-mobile-hotspot%3F