The "Super 8" is the best solution for this. It has 3 slots:
• NES compatibility
• Famicom compatibility
• A "pass-through" SNES slot which can double as a Super Famicom slot if you don't wanna break the blocker tabs out of your SNES
The downside is that the NES part can only use composite video, something common to nearly all NES clones (the sole exeption was the Famicom Titler made by Sharp)