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In this case, two antennas in different directions, is it not the case that if one of them is 90 degrees off the other, the phasing issue will be eliminated because the output from the second will be zero?
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No, because it is not likely that the output will be zero. It is true that the pattern null of one antenna at right angles to the other will attenuate the unwanted signals from the other, but it's not zero. Any interference from the other antenna will cause digital errors. The FEC (Forward Error Correction) has a limit to the number of errors that it can correct. Once that limit is exceeded, the signal reaches the "Digital Cliff" where you have pixilation, picture freeze, and finally dropout.
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But I still have the combiner loss too, right? And is that loss 3 db?
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HDTV primer has led you down the garden path to a false conclusion.
The inherent combiner loss is only about 0.5 dB.
The 3 dB loss that he mentions is the loss that you
would have gained if the two antennas were aimed in the same direction.
Don't confuse the 3 dB loss of a splitter used as a splitter, with the 0.5 dB internal loss of a splitter used as a combiner.
Combining two UHF antennas aimed in different directions sometimes works, and sometimes not. There is no way that I can predict or guarantee the outcome. You must use the empirical approach, which is trial-and-error to find out. I certainly don't want to discourage antenna experimentation. I have always learned something from my experiments, especially the ones that didn't work, because I then had to find out why.
I don't think you will be happy until you try it.