With the "Annabelle" potatoes I harvested the other day, the 2-tuber pot produced almost exactly twice as much as the one-tuber pot, but with these "Juliette", the result is quite different.
The one-tuber pot yielded 863g, which for me is an Average-to-Good result:
The two-tuber pot yielded 1103g, which is slightly disappointing:
Here they are, side by side:
Irrespective of the weight, these are nice potatoes - very clean and smooth. Another bonus is that there were very few of the tiny ones that aren't worth cooking.
My conclusion from this result is that it is still worth putting two seed-tubers in each 35-litre pot. The yield per tuber is not twice what I got from the single tuber, but the yield per pot is better, and seed-tubers are cheap (17p each), so there's no point in scrimping.