Man, that's a tough one. I spent a good chunk of last weekend trying to track down 'Not One But Two' for a proper re-read. I love the premise—two souls sharing one body, all the identity confusion and sneaky scheming—but finding a permanent home for it legally has been a headache.
As far as I know, there isn't an official English serialization on any of the major platforms like Tapas, Tappytoon, or Webtoon. I think it's one of those webcomics that had an unofficial fan translation floating around a while back, which is probably how most people found it, but those sites tend to get taken down. It's a real shame because the art is fantastic, especially in the later arcs when the dual-personality gimmick gets visually creative.
My go-to strategy now for titles like this is to check the original Korean platform, Naver Webtoon. Sometimes they run free original language versions with an auto-translate option, though it's clunky. You might also find it on sites like Lezhin or Toomics under its Korean title, but without an official English license, you'd be stuck paying for chapters you can't read. I keep hoping one of the English publishers picks it up; the plot seems perfect for the current trend. Until then, it's a waiting game, and I'm just bookmarking the publisher's page on Naver every few months to see if an 'English version coming soon' banner pops up.