We just got back from a few days at WDW and had the dining plan for my 5 year old (not the 21 month old) often we had them spilt meals, the only times we didnt' is when my 5 year old wanted something the 21 month old couldn't eat. (even then we should of really, the 5 year old ate more of the babies food then her own.)
The counter service it doesn't mater. At Peco's bills they don't have a kids hamburger meal (just hot dog). I got a double hambuger and gave each girl a patty, as that is all they eat, (not the bun.) It worked out great. BTW the hamburgers there are good, and the topping are great.
My friends had kids the same age. They had more of a problem because they wanted to get each kid there own meal. The thing is they really should of spilt. More often then not you have 2 half meals left over. At Coral Reef the waitress talked them into just ordering one kids pizza, and if they wanted a 2nd she would bring it out in a flash. They did just fine with only one.
What I would do is take turns letting the kids chose what they want. Bring your own sippy cup for the under 3 year old so you don't have to buy a drink. It is just easier not to have 2 bills at the end of the meal.
We saved a lot on the plan, but we wouldn't have ate that much or that expensive if we weren't on it.