As long as you are not on the dining plan, you should have no trouble splitting an adult meal at an a la carte restaurant between two children.
No sharing is permitted at buffets, family style all you can eat restaurants (like 'Ohana or Liberty Tree dinner) or fixed price, fixed menu restaurants (like CRT). Everyone age 3 or over will be charged for the meal at these places. Sharing the all you can eat skillet at Whispering Canyon with someone who did not order it is also not permitted.