From Sesameworkshop.com :

--I would love to know if the two Mr. Noodles are the same person?

There are two different Mr. Noodle characters and two different Mr. Noodle actors. Bill Irwin played Mr. Noodle on the first season of Elmo's World, and Michael Jeter has played Mr. Noodle's brother, Mr. Noodle, from the second season on. This season, keep your eyes peeled for two Elmo's World episodes that reunite the brothers!