This is kind of a simplification of what happens and alot of stuff is left out but here goes.
When 2 signals meet together totally in phase from identical antennas you have 2 times the signal. For every doubling of the signal you gain 3db but due to cabling and combiner losses it will be somewhat less (2 - 2.5db typical).
If the signals meet out of phase there will be some sort of signal cancellation which could cut the signal by as much as half. Half the signal will be minus 3db and the combiner and cable losses still apply so it could be more like minus 3.5 - 4db. The max negative effect almost never happens with directional antennas because it would be difficult to get 2 identical signals coming from different directions with equal strength.
There is also the impedance mismatch to consider which could work for you or against you. When you combine 2 antennas the combined antenna impedance will be about half of what the antennas would be alone. Combining (2) 300 ohm antennas will give you 150 ohms where they combine not counting the interaction of the antennas due to their close proximity to each other. Most antennas are not a true 300 ohms across the whole TV band so there could be spots where the antennas are actually closer to 600 ohms which when combined together would give more than 3 db gain net and other places where it may be 100 ohms which it could make it almost nothing. That's one of the reasons if you look at the net gain curves of a 4221 and 4228 (which is just (2) 4221's stacked side by side) you will see there are spots where the 4228 isn't any better than a 4221 and other places where it's 3db or more better.