Martin Gilbert, of Aberdeen Asset Management, is to focus on client relations and marketing.
Keith Skeoch of Standard Life, will lead on running operations and investments.
The share of responsibilities reflects the merger being forged from a stronger position for Standard Life.
Aberdeen Asset Management, which has been led by Mr Gilbert since 1983, has been in a weaker position since emerging markets declined, and £100bn has since been withdrawn from its management.
Once merged, the company - which has yet to choose its name - will have around £660bn under management, of which £357bn is the most recent figure for Edinburgh-based Standard Life.
The statement issued by both companies explains that a committee of the two chief executives, along with the chairman and deputy chairman, will handle questions arising out of joint leadership.
This is partly to reassure investors and clients that the merged company has clear direction and avoids conflicts. The two men said they have been friends for decades, and go fishing together.
As co-chief executive officers (CEOs) they will share responsibility for core parts of the chief executive role, including the executive committee, developing strategy and objectives and monitoring performance. They will also share communications and the integration programme after the merger takes place.
Keith Skeoch is to have individual accountability for day-to-day running of the business, including investments, pensions and savings, the joint ventures Standard Life has in India and China, operations, finance, human resources, risk and regulation.
Martin Gilbert will lead on international activities, client engagement and business development, marketing and corporate development.
Sir Gerry Grimstone, chairman of Standard Life and of the proposed combined group said: "I am delighted that we have announced these clear accountabilities for the Co-CEOs in the combined business.
"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.
"This blend of complementary skills and experience will serve the company well".
The two Scottish fund manager giants which recently announced plans to merge have set out how their chief executives will share that role in future.