Okay, I was afraid it isn’t that easy :-)
Actually it is related to this question.
I thought I could create a binding on the detail node and within the binding find the “main” node through the link and then load the data from there. This way I wouldn’t have to copy the data to the detail node but just manually update the binding each time the main node changes.
So I think the linkConnected handler is not what I need. However, it seems that I could save the node key in the node data of the detail node and then use diagram.findNodeForKey() to find the main node.