i think it would be a nice feature to be able to define groupings of places (maybe even including difference and even intersection options, in addition to union) outside of projects. that way you could reuse the place groupings. unfortunately, that functionality doesn’t currently exist, and i don’t think there’s a way to duplicate a traditional project, nor a way to duplicate a collection project that you’re not managing either.
if you don’t actually need the project structure, you could just filter for multiple places in the Explore page. here’s an example URL that will return your observations from both Yellowstone and Yosemite: https://www.inaturalist.org/observations?user_id=tesgeorge&place_id=68542,10211 .
(you could get a list of place ids from the file referenced here: https://forum.inaturalist.org/t/use-wikidata-for-place-names-and-wikipedia-descriptions/7702/13 .)