The terrain is really a factor and I think the report doesn't factor that in I really rather have a single antenna for VHF and UHF signals. It makes seems far less complicated to install.
I guess the reason folks say 2 antennas is that Fox, ABC, KCOP and KCAL are all on VHF and on Mt Wilson which is 290 on a compass (NW)
Most of the other stations are on UHF and NE of you (68 compass)
The fact they are in 2 different directions would be why 2 separate antennas sometimes work better. Most of the Mt Wilson UHF stations show negative on the chart which unless you have a big setup wont come in (and even if they do it might be hit and miss)