I got a fruit here and there with one plant, and a whole lot more with two. I hand pollinate simply by poking the flowers each day and then poking others. The first day it seems they don't shed pollen but 2nd day they are powdery with it. Tubas are kind of temperamental for setting, it seems. They want water, but not too much, they want warmth, but not overly hot.
I've had a good 20 flowers so far in 22 and zero sets, even with hand pollination and 2 plants.