The pair will work across a number of WCRS accounts, reporting to the agency's creative directors Yan Elliott and Luke Williamson.
Seftel and Day joined forces two-and-a-half years ago, and have since produced campaigns including Sony Blu-ray "lasers", the Velvet Super Nanny idents, and the "Meat" campaign for BBC 3. The pair were also responsible for the Orange "I am" TV spots.
Elliott said: "It's not easy to find innovative thinkers at a senior level and as soon as we realised their names abbreviated to Larry David we knew we had to have them."
Both Seftel and Day began their advertising careers overseas, with Seftel working for The Jupiter Drawing Room in South Africa, and Day at Saatchi & Saatchi Singapore.
While at The Jupiter Drawing Room, Seftel won three Cannes gold Lions for his work on Nike, before moving to Lowe to work on the Carte D'Or and Skoda accounts.
After Saatchis Singapore, Day returned to London to join Interbrand. He then worked for Exposure and at the Jason Bruges studios before moving to Fallon.