I don't know if this is possible, but here goes... I have a dataset containing three tables - one parent and two children. I have relationships set up between the parent and each of the child tables, but not between the child tables as they have no common fields. When I expand a row, I want both of the child tables to show in the same band. Can it be done?
The windows grid allows this functionality, so an existing UI is there for reference. But i agree it would make some pretty cool markup to create that for the web.
You can however as a workaround show a relationship selecter combo like suggested. There is a Band.ChildBandColumn property, which tells which child band to render.
Hope that helps.
Ahmad