Ahhhh...
Consider, if both antennas receive identical amounts of power and then deliver equal signal power to their respective combiner ports, the combiner is being driven with twice as much power (+3 dB = an increase of power by two times) than that of a single antenna. Sounds great, we should hope that the increase in power may help reception... right? But alas, the splitter/combiner has 3.5 to 4 dB loss between the input and output ports. So, the net gain is between -0.5 and -1.0 dB when compared to the net gain of a single antenna.
There is a way to combine two antennas using transmission line as a combiner:
http://www.kyes.com/antenna/stackluge.html. However, expecting more than 2 dB net gain is unrealistic. Antennas that have balanced 300Ω outputs lend themselves to ganging via 450Ω twin-lead harness rather easily. Unfortunately, many antennas are now equipped with integral baluns which would require some sort of modification to the antenna if one wanted to connect twin-lead.