Yes, you can add two antennas. I had the same issue with my OTA channels. All my channels are within 10 degrees of each other; however when I would aim the antenna I would get all but one. Would reposition the antenna to pick the one up and loose another!
So, I added another antenna and solved all my problems!! One antenna was in the attic, the other I placed outdoors approximately 50ft away from the attic antenna. Positioned the antennas at opposite ends of the broadcast range (i.e., one 60 and the other at 80), ran separate cables to each antenna from my receiver and combined them at the receiver end. Now I get all OTA's at 98 - 100% where as before I only got 85 - 90% and only 4 of 6 channels.
Good luck!