Welcome. The funny thing is with digital it does not matter how much you improve it since the PQ and AQ is going to stay the same. Talk to the vendor or the manufacturer they might have information that at least I am not aware of.
The best case scenario by stacking 2 equal antennas in the same direction is to improve gain by close to 3 dB or in other words double the power level. Theory regarding combining antennas in different directions is quite complicated and practically hardly ever used. Any phase shifting will compromise the result signal rather than help. Two signals coming from different directions will more likely be out of phase therefore the end result will be a reduced signal. But heck , what do I know, like I said maybe the vendor/manufacturer combo knows better.
They essentially are reliaying on the trial and error capabilities of their customers and the fact that in most markets there are not too many digital TV stations and more likely there are a few that at least you are not going to be interested in. Maybe you are lucky to point them in a way that the Stations heavily affected do not interest you , and the rest of the pack although it gets affected , at least not to the point to loose signal.
The pro advice would be to have a rotor with a single antenna rather than this monster or frankenstein creation. Funny that only chineese are bulding this ones while the major antenna historical manufacturers have none of this. I just wonder why eh?
