Patrick,
I recently did this on a form I was working with and it totally messed me up and I was up till 3am trying to figure out what happened. I think there are some bugs on SP1. For instance on my main schema I have a repeating group. I add a secondary schema with a single group. I add the group from the seconday schema as a section onto the form. Then I put the repeating group from the main schema as a repeating table into the section.
Lets say I then go to the repeating table properties, make the table a detail table by going to the Master/Detail tab and then check the Copy Key from Selected Master when Inserting Detail. Then I save and close IP and then reopen it in design; when I go back to the table properties, the checkbox is unchecked.
I have very similar issues as well with repeating sections, everything just get hosed...
Jason
http://www.k2distillery.com/