This is not often used, and rather difficult to find any information on.

Of course it is right there in PeopleBooks, if you read carefully.

But here is a copy-paste example:

&MyRowset = CreateRowset(Record.PARENT, Field.CONTROL_FIELD, Record.PARENT_SIBLING, CreateRowset(Record.CHILD, Field.CONTROL_FIELD, Record.CHILD_SIBLING));

Note that the 2nd record listed there is supposed to be a Related Display Record, and Field.CONTROL_FIELD its control field. I’ve found you can create sibling Records like the above, and just use any field for the CONTROL_FIELD.