As shown below, we have to make sure that they are separate from each other and have enough food directly in front of them. Otherwise, Phoebe (she tends to do this a little more than Hector) will see Hector eating food in her periphery and will want to eat the same food since he seems to have more food than her. She will try eating food that Hector already has in his mouth and will accidentally bite his head (anyone that has watched a tortoise walk, eat or climb knows that they do not have good coordination). Another scenario that has happened several times is that Phoebe will start eating Hector's food and will completely inhibit him from eating at all. This is one of those time when Hector shows his personality. He bites Phoebe, and he's not too picky about which body part he attacks--whatever is closest is good enough. Fortunately, this is a very rare occurrence.
We did pretty well this morning at estimating how much they would eat. Some days are better than others because their appetite will vary somewhat. Even these last leaves Hector would eat if we fed them to him by hand (it's quite hard to eat something flat that's on a flat surface when you don't have hands or even lips--oh the trials of Chelonian life). Phoebe might eat them too, but Hector was the only one still awake (Hector eats more and longer and Phoebe goes to sleep first, generally speaking) while I was leaving for work when I took this picture.