The pair join from Wunderman, where they were the joint associate creative directors, and where Bell was also the head of art.
The hiring ends a four-month search by CMW for its new creative heads, following the resignation of the creative director, Bob Nash, in June.
At Wunderman, Bell and Hunter's clients included Orange, Land Rover, Rolls- Royce and British Airways. Their work on Rolls-Royce won a bronze Cannes Lion in 2007, as did a campaign for Green Tomato taxis.
The pair first formed a creative team when they joined Wunderman (then Harrison Troughton Wunderman) in 2004. They were appointed as creative directors in 2006.
Martin Nieri, the managing director of CMW, said: "We spent a great deal of time looking for a team with the drive to build on our recent growth and push our creative offering forward, and that's what we have in Jamie and Iain."