Standard Life will take over two-thirds of the combined group in the deal.
London: Standard Life chief executive Keith Skeoch will oversee its day-to-day running after it merges with Aberdeen Asset Management, while Aberdeen boss Martin Gilbert will handle external matters, the companies said.
Analysts have expressed concern that the co-chief executive structure proposed by the firms when they announced an 11 billion pound ($13.6 billion) tie-up two weeks ago will be unwieldy.
"Both boards have thought carefully about the key responsibilities and believe that the proposals play well to Keith's and Martin's respective leadership strengths," Gerry Grimstone, Standard Life chairman and chairman of the proposed firm, said in a statement on Monday.
"This blend of complementary skills and experience will serve the company well".
Mr Skeoch's responsibilities will include investments and pensions, while Mr Gilbert's will involve marketing and distribution, the two firms said, adding that a chairman's committee will be set up to ensure co-ordination is effective, chaired by Grimstone.
The merger values Aberdeen at 3.8 billion pounds in a deal in which Standard Life will take over two-thirds of the combined group and both sets of company directors will split power on the board.
Analysts say the merger is defensive, as active fund managers face increasing regulatory scrutiny and competition from lower-cost index tracking funds, and could lead to outflows from both firms.
Following an initial rally, the shares of both firms have fallen below their closing prices on March 3, shortly before the deal was announced.
Standard Life's shares were trading at 361.5 pence at 1246 GMT (6:16 pm in India), up 0.5 per cent on the day but down 4.5 per cent from March 3.
Aberdeen's shares were at 267.7 pence, up 0.4 per cent on the day but down 6.5 per cent from March 3.