Bad idea be half impedance plus get comb effects.
This ^
Do a Google search for images showing interference patterns. You'll see where the wave add and subtract from each other. What you're looking at is what happens when a single frequency interacts with itself, and you're seeing just the 2D version. This is a flat field. Sound is far more complicated. You have a broader range of frequencies, and they spread over a 3D area. There's also the relationship to do with the listener's position based on distance, angle and height.
What will happen is the sound will lobe. That is to say that there will be positions in space radiating out from the speakers where some frequencies are loud and others are dead. This will change too as a listener moves. The result will be that no two listening positions will give the same sound. The audio field will become very inconsistent compared to a single speaker.
The other point is what happens to the amp when it is running with a lower impedance speaker.
The Denon AVR-X series already has a bit of a reputation for running hot at normal listening levels. Wiring the two speakers in parallel will lower the impedance, and that means that the amp will be asked to deliver more current. In effect then, it will be working harder. That means it will be getting hotter.
The alternative is to run the speakers wired in series. That will decrease the current drawn, but at the same time also reduce the volume that the centre channel can reproduce. In compensating for that with the speaker level control you'll get back to square one because you're running the channel louder to make up the difference, but at least you're avoiding the danger of the lowest dips in the speaker impedance curve presenting the amp with a demand for a huge amount of current that it can't really provide.
