I bend sides for guitars or ukes at the rate of about 1 per week. I use a heat blanket, and make my bending forms the thicknes of the sides smaller than the outline. Remember that it is the heat, and not the water, that makes the sides soft enough to bend. I bend between 2 sheets of stainless steel, which I get from a commercial kitchen supply place for about $10 a pair. Put the probe from the thermometer between the blanket and steel, as it can stain. If I am using a really high oil wood, I do wrap in brown paper to avoid staining.
I got my bender from blues creek guitars, and block it up to handle ukes. I just finished a kasha baritone/ cutaway in rosewood, and had no problems with the bend. I can't imagine why you would need to bend both sides at once. I bend between 250 and 290 F
You can see me bending a rosewood side for a guitar at my web site
www.hastingsmade.com under the media section. I built my first 2 ukes just using an iron, but it is so easy to make bending forms, that I now have forms for all the ukes I build. I just make a single 1/2 pattern in some high quality 1/2" plywood, using a band saw, and spindle sander. Now take this pattern, and cut 3/4" plywood close to the line, and then use a flush cutting router bit to make all coppies exact. for a uke I make 3 pieces, screm together using 1 1/4" spacers to get a bending form just over 4" wide.
Hope this helps, I find it is plenty of work to bend 1 side at a time, and do it acurately!
Jack