Nighthawk MR1100 one omni plus one directional external antennas?

I'm in a semi-rural area and bought a directional Yagi antenna to try to get a better signal for my MR1100 Nighthawk on AT&T. Turns out I'm doing more tower-hopping than I expected and my window for returning the directional antenna has closed. If I bought an omni antenna and added it to the MR1100 so it had an omni AND a directional, each feeding its own TS9 port (presumably the omni would use the left port), would that be any better than the omni alone? I see there are critters called "combiners" out there; would that be necessary? I'd keep the directional pointed at the tower that I'm most often receiving from. Thanks for your input!