2 x 1961 1S data better than one 1723 2S ?

Your theory is pretty much right, the +3dB vs +6dB increase will depend on the distance between them and at what frequency though.

So looking at the data, you will get about the same output from 2 x 1961 Subwoofer 1S as you would from a single 1723 Subwoofer 2S considering the measured CEA response. With that said, the 2S gets a little penalized by not having the two woofers on the same surface which gives us a bit less output on a ground plane measurement vs if you would measure it in room. Same goes for the 2V. It means we have to measure 90 degrees to each woofer where they sum up.

Apart from pure sound pressure, which is only what CEA indicates, the 1723 Subwoofer 2S is an even better-sounding subwoofer. If APP control is important to you, then that is another difference between the two.

The dual driver designs couple a little better to the room. They have a complete lack of sideways movement due to the push-push configuration. The drivers on 1723 itself are larger but actually has slightly less linear excursion also meaning a shorter voice coil, with less wire spun around it – lower inductance which translates to better transient response and they move less for the same output. The cone itself is also stiffer, with fibre-glass backing on 1723 meaning it behaves even more so as a piston in its usable frequency range.

All are small things, that sum up a sonic difference in the end.

The 1723 Subwoofer 1S has the same driver design as the 2S, so that applies, the difference still is there for the push-push configuration of the dual drivers as well as better room coupling for the dual driver version though.

The best way for you to have more of an educated guess to what will work better in your room is to download REW and use the room sim function, which lets you set up your listening position and room layout so you can try different locations with one or two subwoofers to see the theoretical effect your room will have on the low-end performance.

If you are free to place two subwoofers wherever you want, that will enable you to get the best frequency response in your listening position. After all, it is a game of evening-out room nodes which will always be there in any given room. If you are stuck with one or two locations on the other hand, then it may not give much benefits by going dual subwoofers depending on where those are.