Iris said that the appointments follow increased demand from current clients and its recent win to handle the advertising and integrated marketing for Volkswagen Commercial Vehicles.
Gage and Owen will work alongside Paul Kenny, who has been at Iris since leaving Ogilvy in 2007 and will retain his role as the third head of planning.
The trio will drive planning across the agency's five 'houses' which each service up to five clients as part of Iris' revised agency structure, which launched in October.
Owen joins from TBWA and has previously held senior planning roles at Glue and 180 Amsterdam. He has led digital, strategic and media planning projects for clients including Adidas, Bacardi and Toyota.
At Iris he will be the global lead planner on Adidas' running account and sports equipment brand Polar, as well as leading the London office's planning operations for clients including Sony and Coca-Cola.
Gage joins from Proximity and brings experience of digital, communications and data planning. He has worked with clients including Abbey, Renault, Nivea, COI, Diageo and VW.
In his new role he will lead planning operations for Sony Ericsson, Bacardi Brown-Forman, the COI and Gala Bingo.
Kenny will lead all planning initiatives for Sony Ericsson's global account, as well as for London clients Philips, Disney, ING and Shell.
Sam Noble, global executive planning director at Iris, said: "Both Paul and Duncan have proven track records in inspiring unique ideas that get people talking and drive real value into client businesses.
"They are a significant next step on Iris' journey to put world class, multi-disciplined strategic capability at the heart of our rapidly growing, international, independent network."