Richard Gamble of Royal will be group chief executive but Roger Taylor of Sun Alliance will have the senior role as executive deputy chairman and chairman of the management board.
The arrangement is part of a carefully balanced reorganisation that sees the departure of one Royal director, Roy Elms, and one from Sun Alliance, Ian Trotter, both of whom will be consultants to the company.
There are bound to be questions over whether the emergence of the two most powerful figures in both companies in similar positions at the top of the new organisation can be a lasting arrangement.
Mr Gamble, 56, will have overall operational responsibility for the group, running the management team, and will report directly to Mr Taylor, 54. Mr Taylor will be responsible for the world-wide strategy of the merged business.
Asked to define the two top executive roles, Mr Taylor said: "The board has delegated executive responsibility to me."
Above both men will be a new non-executive chairman, to replace Sir Christopher Benson, the chairman of Sun Alliance, who will remain as interim chairman of the merged group until the new person arrives.
An acquaintance of Roger Taylor yesterday described him as "an insurance man through and through." Mr Taylor joined what was then the Alliance straight from school, in the West Midlands. He has stayed with the company ever since, and was the driving force behind its last big acquisition. In 1984 it bought another insurance company Phoenix for pounds 400m, and achieved 18 per cent cost savings in as many months.
Richard Gamble, by contrast, trained as an accountant and has had a varied career, including since 1976 the finance director's role at Northern Telecom, McDonnell Douglas UK and British Airways. Mr Gamble joined Royal from BA in 1989 as group finance director, and was seen as a good outside figure to pull together what was regarded as a somewhat sleepy company with severe problems in the US.
The new group will have four executive directors - Mr Taylor, Mr Gamble, Arthur Hayes from Sun Alliance and Paul Spencer as group finance director from Royal.
The non-executive directors have been carefully chosen with equal balance from both original boards and include the Duke of Westminster from Sun Alliance. A number are also quitting.