ryoung, here's the picture from your quote
Fully supporting entire tire footprint is required IMO, however requiring both duals to be blocked I dont. Few of us are anywhere near capacity weight wise (static load) that a single tire will end up with more load than rated.
I prefer and try to block both- but I only carry so many blocks..
so as often as not, only block one of the duals. If storing long term- Id block both but for few days I dont believe I'm damaging tires.