This is actually a really neat twist on Minesweeper and I think the multiplayer aspect looks like a lot of fun (though I've only played it solo). There's a lot of polish here, and I especially like that it's smart enough to end the round early when it's no longer possible for the other player to win. I wonder if it might be a good idea to have an optional move timer that forces a random move to prevent a player from taking too long, to encourage some quick thinking. A way to move the cursor more quickly would be handy, too (maybe when held after a delay, or by holding B).
One major issue that should be addressed is that the second player controller is susceptible to DMC DMA glitches. Simply using repeated reads is not sufficient; you need to do repeated reads of one controller until it matches, and then again for the other controller. (Or, alternatively, use OAM DMA synced reads, which I prefer.) See this thread for more details: https://forums.nesdev.org/viewtopic.php?t=24952