While some people do use both the diet and stimulant medication,
in almost all cases they can use less medication than expected.
We have never seen any research done on this phenomenon, although one old study did point to it (Williams 1978) See a short discussion of the Williams study here.
Using a computerized test of attention called the TOVA, one of us (Shula Edelkind) did an undergraduate independent study in college in 1998 which found a clue:
The TOVA reports 4 parameters, two of which are called "cognitive" and the other two are called "behavioral." All four must be in the normal range for the child or adult to behave and learn normally. When used to monitor the effects of Ritalin on a child, the "cognitive" parameters rise to normal first, according to the TOVA company. Under increased medication, the other two rise.
However, during the unpublished independent study, it was noticed that when put on the Feingold diet, the "behavioral" parameters of each child rose to normal first, followed by the "cognitive" ones. If using both diet and Ritalin, theoretically, the diet would raise the "behavioral" parameters while the medication would raise the "cognitive" ones at the same time. Therefore, it makes perfect sense that less medication would be needed for the best effect.
Some of our members find that during high pollen seasons, their allergic children need more than the diet alone - some benefit from a very tiny dose of medication during that time. Unfortunately, no studies have been done on the relation of allergy to the need for medication, or why medication would be needed only during those times, but if this seems to be a problem in your family, work with your doctor to achieve the best possible results for yourself or your child.