I agree with Vortex that if you are taking analogue out of our DVDP, I'd think the DVDP is programmed in a way so that the sound is necessarily 2-channel and you do not have to worry about downmixing.
Downmixing is relevant when you are taking digital out from your DVDP and putting your digital cable (Coaxial or optical) into a 2-channel DAC. This is what I do. I first downmix the 5.1 or whatever multichannel signal in my DVDP into 2-channel (stereo) digital signal and then send this through a digital cable to my 2-channel DAC. The analogue outputs of the DAC then go to my 2-channel amp. In my DVDP there is also an option of having this 2-channel digital audio in simulated surround mode. I use that too. Whenever we watch movies at home, we do it this way while an HDMI cable connects my DVDP to my LCD TV. The HDMI cable also carries audio, but we turn the volume of the TV down or can have it too at moderate volume (using it something like a centre channel). But the sound of my TV cannot match my DAC+Amp combination, so most of the times we keep the volume of the TV at zero position.
In your case, as explained above, just connectiong your DVDP with your 2 channel amp by analgue RCA stereo cable will give reasonable sound if your DVDP is decent (the denon 1940 is decent I believe).