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.