The max output will be similar between these two solutions. With dual 1961 Subwoofer 1S, you also get the option of moving the subs to the locations in the room that give the best integration, which you don’t get with a single subwoofer.
With that said, if you are after the higher sound quality, and listen to music a lot, the 1723 Subwoofer 2S is a superior sounding unit – although the 1961’s are no joke either.
My personal choice would be dual 1961 Subwoofer 1S as you will likely end up with a better frequency response in the listening position.
Even with careful placement, a single subwoofer will not have an optimal frequency response at all listening positions in the room.With properly located dual subwoofers you get a smoother frequency response at more listening positions in the room, with less potential for obvious peaks/nulls in the frequency response.
I suggest you to go with dual 1961 1s if you can’t buy a second 1723 2s in future. Another viable alternative is to get a 1723 1s now and a second 1s in future, so you’ll save a bit of money compared to the dual 2s option.
I would always go for dual! As a single sub will have room modes and the bass will not be consistent between the seats.
Example here (Green and Red locations individually have awful dips in their response, but combined they tackle each others weaknesses and help to get a good flat responce):
This is both subs combined (before proper EQ):
Going will dual would help to solve those issues. BTW what receiver are you using in your setup? This makes big difference since you want one that can EQ each sub individually instead of just having a simple Y splitter built it.