Monster Lair is in the series, but it's a platform/shooter as opposed to a platform/rpg.
On the plus side, it's cheap, two player and a CD.
I can't think of many Duo games like Dragon's Curse. Legend of Hero Tonma isn't too far off. It's definitely more of an action game but it does have bags of gold and a medieval flavor. The Japanese version is cheaper than the TurboChip and doesn't require much reading. Tiger Road lacks the RPG elements, but it's a great platformer and saves your progress.
Get yourself a used TurboGrafx so you can play US hucards. It's a nice companion to the PCE Duo.