If you don't mind Japanese games, you can also get Cosmic fantasy 3 for around $10 on ebay usually, and it will play on a US turbo duo. Neutopia 1 and 2 are pretty nice zelda clones (think nes version of zelda 1, both hu-cards). They fluxuate in price. Double dungeons and dungeon explorer are pretty fun hu-cards, because they support multitap. Dungeon master was released in the US for the duo. AD&D Order of the griffen on the hu-card. Those two games, most people shun, because they are quite different than a lot of your typical rpgs. They are games you might expect to play on a PC, with a mouse, rather than a controller.
I really don't know many more US releases, but there are a ton of japanese duo cds that will work on an american duo. If you have no fluency in the language, it will take a couple hours to learn the menus and how to get around, fighting, spells, etc in cosmic fantasy 3, and I guess each other game has its various demands of Japanese, some you probably could never learn without knowing it, while others, it's easy to intermediate.
My key for getting really far in cosmic fantasy stories for the sega mega cd, with no Japanese knowledge, was to do what you'd normally do in any dragon warrior type of game: Talk to everyone in the villages, explore everywhere, level up, buy the best weapons and armor with each new village, and if you get stuck somewhere in between, go back and talk to everyone again, and just make sure you've explored everywhere, and you usually achieve the desired results. The only minus to that I see is that you miss most of the story line, if not all.
