SOHO MK3 with 2 external antennas?

I use the MK3 in our RV (Airstream so coverage outside is not too good). I want to add an external antenna (MobileMark or such). I know I can get those external antennas with 3 WiFi connections to use all the connectors on the MK3 but how would only 2 connections work? If I get an antenna with 2 WiFi connections (they are cheaper), would the MK3 still function on WiFi as WAN? What would I hook up on the ‘empty’ connector?