Just for kicks, I loaded a Super CD 3.0 rom onto my flash cart and plugged it into the Duo to see if it would work. I played a little Dracula X and Gate of Thunder, visually it was identical to playing on the Duo normally.
I was actually testing to see if I could use the flash cart as a system card, since I was wanting to get a cheap TG 16 + CD combo for mod experimenting, but the US 3.0 cards seem ridiculously expensive. For the record, Super CD Rom games work, Arcade Card games do not.