As you start to reveal the shape you desire, you slowly take little chips of wood away. But the clue when carving simultaneously is to do just little at a time! When you've done a section on the first piece just lay it down and pick up the other piece to do that same section. Again taking away as much (or should i say as little?) as you did on the first piece. When you think you're done, pick 'm both up to compare and when you feel one is a bit "out" of the other, this is the time to make sure you correct that. Only when you are happy that they are equal, can you proceed to carve a bit further on the shape. If you're not careful, a slight change in shape will become more and more visible as you reach the end. So be sure to take little steps at a time because then it's far more easier to keep both pieces the same shape. So you can end up with twins :)
This is the way i proceeded on these two dolphins. I mentioned the photo album before, but in it you can actually see and follow the step by step progress of what i just described. Still a lot to do but the heads are roughly finished now. Although I'm proud to say they do look a lot alike... I can easily tell them apart. I've held them up close for so long that it almost feels like being a Mom of twins. Every one is confused who is who, except their Mom. They both have their own character :D
The tools I've used so far are nothing fancy. My two favorite ones are a scalpel #3 with a straight blade #11, and the one i use most, scalpel #4 with blade # 23, all from Swann Morton. The last one has a very nice curve to the blade, that makes it really easy to cut and carve through hard woods. Beside these knifes that do the most part, I used a few gouges, ranging from 0.5 till 3 mm.