If the one antenna will not receive channels 21 thru 19 because the distance between 21 and 19 is too wide. Then install 2 separate antennas , the HD7694P pointed at channel 21 and a Antennas Direct - Clear Stream 2 pointed at channels 19 and 49. A preamp will not be needed with Clear Stream 2 , the signals are strong enough for 2 Tv's. The installation will be 2 separate Antenna systems , Do Not combine the the 2 antennas on to one coax. You will have a Remote Control A/B antenna switch at each Tv location.
http://www.antennasdirect.com , has remote control A/B switches and so does Radio Shack and
http://www.solidsignal.com , and
http://www.amazon.com ,
http://www.3starinc.com