I think the idea of groupx and fieldx is for something simpler than you expected at the moment, because the way seblod 'render' the page is bit different and it is not 'nested'

Yes Viktor, this is what I saw. The other way would be to 'attach' contents with a multiple dynamic select field. 

This way we can have a groupx containing IDs of other contents but we loose the on the fly dynamic creation interface. We would then need to create the first contents and then only after create the second content referring to the first one.

It is also more complex to display the first content fields in the second content since they are now only related and not nested as with the groupx.

thanks

cyril