People who pick up the Final Fantasy X|X-2 HD Remaster on the Nintendo Switch will need to keep the cartridge handy to play both games. While only Final Fantasy X is on the cart and Final Fantasy X-2 is a download, you must have the cart in at all times to play either game. (Thanks, Reddit!)
Final Fantasy X-2 is not available on the cartridge for the western version of Final Fantasy X|X-2 HD Remaster. When people buy a physical copy, it includes a voucher with a download code for the sequel. Because of the nature of the game and how it is essentially DLC for the compilation, people need to have the cart inserted at all times.
As an aside, the Japanese and Asian versions of Final Fantasy X|X-2 HD Remaster for the Nintendo Switch actually give people both games on the cart. These releases also have the option to have English voice acting and subtitles and are region-free.
Final Fantasy X|X-2 HD Remaster is available for the Nintendo Switch, PlayStation 3, PlayStation 4, PlayStation Vita, Xbox One, and PC.