Arcade Card Duo is for the Duo consoles and nothing else - it contains less memory as the Duo's don't need it/have their own memory.
Arcade Card Pro is compatible with all consoles and contains extra memory needed by setups such as the briefcase setup/SuperCD-ROM2 setup.
One correction, SuperCD-ROM2 DOES contain the extra memory can be used with the Arcade Card Duo card. To recap:
- Arcade Card Pro: Briefcase CD-ROM2 (with original PCE, CoreGrafx 1/2, LT, SuperGrafx), TurboGrafx CD-ROM (region modded or with converter card)
- Arcade Card Duo: Duo, Duo-R, Duo-RX, Super-CDROM2 (with original PCE, CoreGrafx 1/2, LT, SuperGrafx), LaserActive
Details here:
http://pcenginefx.com/main/nec_compatibility_guide.html