Unless there is a balun between the two antennae and the input, then the impedance at any frequency will be changed. I was going to say "halved" but my knowledge of rf is insufficient for me to be sure. If you put two resistors in parallel then the total resistance is halved, but inductance involves more complicated math. This suggests that at any given frequency whatever the impedance of a single antenna is will be halved when they are in parallel. They are too close together to be a phased pair but likely the directivity at any given frequency will be likely changed at least slightly. Also if they are wired out of phase then the antenna will act like a transmission line and the sensitivity will drop way down unless they are at resonance for the distance between them.Maybe it's just attenuating the signal for the sensitive receiver. Sometimes that can give the effect of "cleaning up" a signal.
It would be interesting to note what systems you thing this made a difference on???I'm not much on the engineering side,but it does work,it appears to me that this setup has cleaned up the reception of some of my channels
Well from what I've seen so far you don't have a basis for making that claim. It is clear that you yourself honestly think it works, but you have shown no actual evidence that it does. And you have cited no theoretical argument for why it would work.It works, so why question it? I do it with different antennas, no problem.