Why take two classic consoles into the den when you can use just one? The SG/FC Dual Action Video Game Console offers two cartridge slots, one for Nintendo NES games and the other for SEGA Genesis titles. A switch flips between emulation modes.
Two joypads are provided, together with RCA composite video cables (that support stereo sound in 16-bit games). Best of all is the price: from around $32.99 on eBay. Obviously you'll have to pick up a few games to go with it, but there's plenty to choose from (on eBay again) for just a few bucks.
Apparently a new version of the console is about to launch, with three slots and compatibility with NES, SNES and Genesis titles. No specific release dates, though.