It's a good question. I don't think anyone's ever put together a definitive list. I don't even remember any specific examples off-hand, just that some games would be mentioned as being dual releases in Nintendo Power coverage. (This happened a lot more with NES + GB or SNES + GB dual-releases, but these are obviously not "source ports" but rather ground-up rewrites, so not interesting here.)
Of course, some of those are sequels that just happen to share a name; some are ground-up-rewrite-style dual releases; and some are just unofficial ports — people trying to recreate a SNES game on the NES or vice-versa, without access to the original code. But there are likely a good few in there that were SNES-enhanced-NES-code dual-releases.
If someone here is a gaming content-creator, there's a very unique deep-dive essay waiting to be written here, about how these games work. (Retro Game Mechanics Explained, I'm looking at you.)
But that doesn't mean that we can't figure it out! Grabbing the No-Intro databases (https://no-intro.org), and then intersecting the romset names between the NES and SNES lists, gives me this list: https://gist.github.com/tsutsu/0a9d7dbaa3cb7bb1929b31d7bc878.... About 82 games!
Of course, some of those are sequels that just happen to share a name; some are ground-up-rewrite-style dual releases; and some are just unofficial ports — people trying to recreate a SNES game on the NES or vice-versa, without access to the original code. But there are likely a good few in there that were SNES-enhanced-NES-code dual-releases.
If someone here is a gaming content-creator, there's a very unique deep-dive essay waiting to be written here, about how these games work. (Retro Game Mechanics Explained, I'm looking at you.)