The pair, who will report to the executive creative director, Ben Priest, will enter the agency at board level.
The pair will work on creative briefs across all of RKCR/Y&R's clients.
Priest said: "We have one of the best client lists in town and I want them to come in and gorge themselves on the list.
"I have been following their careers since I became creative director and they not only have a fantastic reel, they also do great print work."
Chalkley and Simons began their partnership three years ago at St Luke's, where they worked on the Clarks account, creating the "life's one long catwalk" campaign as well as the "Swedish mafia" work for Ikea.
They moved to O&M in 2004, where they worked on numerous projects for Ford as well as creating the Harrods "stitch that" poster, which won the Readers' Award at the Campaign Poster Awards in October.
Chalkley said: "It was an eye-opener moving to such a big agency from St Luke's. Rainey Kelly is going to be different again. It's good getting back to somewhere that generates a lot of work at a quick pace and where we can start writing ads again."
The agency has also hired the digital creative team Tim McNaughten and Nick Flugge, and the junior creative team Joe Fitzgerald and Kim Hutchison.