The help pages are generally pretty awful, and many appear to have been written by non-English-as-a-first-language speakers.
One thing to keep in mind is that the combined shipping rules DO view the weight of the heaviest item as the main weight, and then all other items are additional items.
I don't think it's possible to do what you want, since there are no calculated rule options other than don't either combine all weights or not combine weights at all. (there is no "ignore all additional item weights" option)
You might be able to create a "combine weights and subtract N ounces for each additional" rule for a specific item where weights are combined, but weight of each additional item after the first is subtracted, and an $X per item handling fee is added, but that has problems.
That flow is like this:
select "Combine all item weights and subtract an amount" and for each additional subtract N ounces (where N=item weight). Then on the next page of the dialog, "Specify a handling cost for each additional item" and then "Add $X handling for each additional item"
Main problem is that the rule would be specific to a particular item with a specific rule profile defined for each item and its weight. (and when I tested things out, either the rule itself was ignored completely or the handling fee part was ignored, and all I saw was shipping amounts for straight combined weight - didn't test enough to determine exact failure mode).
