What's the reason for matching two user to the same group?

If you want to make sure both groups will use the same permissions, add a separate database.

Example:

CREATE ROLE [App_Agents] AUTHORIZATION [dbo];

EXEC sp_addrolemember N'App_Agents', N'DomainA\Agents';

EXEC sp_addrolemember N'App_Agents', N'DomainB\Agents';

And assign the permissions required to the role instead of the user.

Make sure the windows user groups will not have individual permissions.