You have to throw one bowl and wait for it to dry partially, the leather hard stage. At that stage it can support the weight better. Then you flip it over on the wheel, center it, scratch up the bottom so the new clay will stick and you put a lump of clay on top and then delicately center that. It is really hard not to let it collapse because if it gets too wet it will. Also if you put too much pressure on the new piece of clay it can make it collapse too.
I sort of had problems with that. What that picture doesn't show, is that there's actually a crack on the bottom of the bowl where I had to fix a part that broke through (which means I can't really use it for icecream, but I can use it for dry things - like m&ms!) The only thing that I can figure happened was when I was making it, I got the joining piece too wet and the pressure of me throwing it caused the first bowl's bottom to bulge out. Not knowing what had happened, I shaved off too much and it got too thin.
It's definitely a delicate balance - which is why my teacher made us do it this way, because it's a lot harder.
The easier way to do it is to just make 2 separate bowls and join them off the wheel.
Also, I think that crack is from firing it too much as well. I had the break fixed so that it wasn't noticeable but I had to put it through an extra firing because my colors weren't right after the first one and unfortunately....firing makes clay shrink, so the crack reappeared.
It's not the first time I've had something crack or break. If only you could see the underside of that octopus!! Those legs were a pain!!