Who are these people who remove some elements from their antennas?

The larger elements are for reception of lower channels. Do you have any VHF-Lo band channels in your area? That's 2 - 6. FM is right above 6, so these pick up FM, too, which could be its own problem.

Antenna amplifiers (you did not mention these, but anyway...) amplify all the signals in their frequency range that you can amplify. If you have no low-band VHF signals in your area, but, say, 30 FM stations, those FM stations will be amplified along with all the signals you want. (FM stations can be trapped out, that is, attenuated, too.)

Anyway, say you are running a bunch of stations from 7 on up through your system and the FM stations are so strong and plentiful that their signals distort in the antenna amp. Distortion gives rise to signals that are multiples of the original wavelengths, so FM could cause interference with your UHF reception on some channels, actually making UHF appear weaker.

The problem with two antennas is that both antennas receive all signals from the directions they are pointed at. If any channel hits both antennas with signficant phase difference, due, say, to reflections coming from an antenna not pointed at the station, these signals will mix and will attenuate or at least mess with the better signal. It's very unlikely that you will improve the signal of a channel by using two antennas pointed in different direction. With one antenna, you get what you get, which might be a clean strong signal or a weaker signal bounced off a mountain, but you only get that one signal, so it can be better.

I write "may" and "can" and "might" because all of this is subject to experimentation in your area. Years ago I did antennas for a chain of retails stores. In one location, moving five feet to the left absolutely killed channel 5! In another location, I had to back the antenna up near a 15 foot high brick wall parapet to kill reflections that were coming to the back of the antenna, creating ghosts (analog days).

...which brings up another thing. Most antennas receive almost as well directly onto the back of the antenna as they do onto the front. It's the sides where reception really falls off.