The duo will become the creative directors, reporting to Levy. By bringing extra experience into his creative department, Levy hopes to channel more resources into Joshua's integrated activity.
Christodoulou and Storey first joined Brann London in January 2000. In August the following year, they left to join a start-up agency backed by HHCL & Partners, but this project was aborted.
The pair were then reappointed by EHS Brann to help the executive creative director, Patrick Collister, bridge the gap between his above-the-line background and the agency's below-the-line remit.
Christodoulou and Storey have also worked at Craik Jones Watson Mitchell Voelkel, FCA!, OgilvyOne and HPT Brand Response.
Christodoulou said: "We are excited by what is going on at Joshua and decided that now is a great time to join."
Levy added: "I am keen to consolidate our position as the best integrated agency in town and Brian and Xanthos will help me do this."
Joshua has greatly bolstered its integrated capabilities this year, employing 30 extra people to this end since January.
The agency has forged a closer working relationship with its sister outfit Grey since the latter appointed Garry Lace as chief executive last year.
The two agencies have collaborated on the successful Visa and Air Miles pitches.
In addition, Joshua has won extra briefs from Nat West, Coca-Cola GB as well as new business from Insight Investment, Norwich & Peterborough Building Society and Help the Aged.