Whenever I do a Demo I do several practice pieces before hand because if you are going to make a mistake it is better made in you workshop than on a demo with the World and its Mother watching.
It also helps you with the timing of the Demo it has to fit in with the start ,break and finish time.
I have just turned these two in one and three quarter hours complete with coloring and polishing this means my timing will have to be spot on.
If the worst comes to the worst as long as I do most of the work on both and only do the coloring and polishing on one I will be OK.
These bowls are approx 6" diameter