A Safari extension that sets one page zoom when your iPhone Duo is folded and another when it's open.
Open the DuoZoom app and tap Open Safari Extension Settings. Turn DuoZoom on, then set All Websites to Allow. DuoZoom can only change a page's zoom on sites it's allowed on.
In Safari, open the page menu to the left of the address bar and tap DuoZoom. Tap either display to choose its zoom for the site you're on, or use Every other site to change the defaults. Under In landscape, you can give either display a different zoom for when the phone is sideways.
Safari extensions can't read the hinge, so DuoZoom goes by the size of Safari's window. A phone-sized window gets your folded zoom and a larger one gets your open zoom. That means a split-view window on the open phone is treated as folded, and on iPad a full-size Safari window uses your open zoom.
Check that DuoZoom is allowed on that site in Settings › Apps › Safari › Extensions › DuoZoom. Safari's own pages, like the Start Page, don't allow extensions. If a page opened before you turned DuoZoom on, reload it.
Questions or problems: open an issue on GitHub.