Here is a summary of why dual members are unable to serve past the group level.
Dual Member History – This gives a history of the policy about dual member service and a page of frequently asked questions. It shows the policy has been in place from the beginning, was recommended by Bill W, and has been reviewed many times over the years by the World Service Conference and the Policy Committee of the WSO. Each time they have reaffirmed the policy.
Dual Members in Service Policy This is a memorandum from the WSO to answer questions including whether excluding alcoholics from service is a discriminatory policy.
Letter–9-15-1971–Additional Dual Member Background Information view and
Letter–10-16-1968–Additional Dual Member Background Information.pdf view
These are copies of old letters sent as replies to members about the questions about dual membership service.
The Al-Anon/Alateen Service Manual (pages 78-79 in 2010-2013 edition) says:
Because of the unique nature of both programs, Al-Anon/Alateen members who are also A.A. members do not hold office beyond the group level, as these positions could lead to membership in the World Service Conference (WSC). The need to focus on the Al-Anon program and the possibility of a conflict of interest at Assembly and world service levels led the WSC to its decision not to seat Al-Anon/Alateen members who are also members of A.A.
For these reasons A.A. members do not serve as Group Representatives (GRs), District Representatives (DRs), Area Coordinators, Newsletter Editors, Delegates and/or any of their alternates, nor do they sit on the WSO selected committees, whose Chairpersons can be Conference members.
So the reasons are: the unique nature of both programs, the need to focus on the Al-Anon program, and the possibility of conflict of interest at Assembly and world service levels.