These two photos show an ambiguous image - how apt for Parmeliopsis ambigua! Both might show a single thallus; but if so, part of the thallus is deficient in usnic acid, leading to the grey coloration. The alternative interpretation is that the thalli are composite; the greenish part then is Parmeliopsis ambigua, the grey part Parmeliopsis hyperota (see below). Sadly, I didn't sample them as I didn't realize how strange these thalli are till I was already back in the Netherlands...