First off I must say that I loved the first Game Brains episode, exactly because of what @benheck said in the begining: It's not like simply putting two systems into a single box, but using components of both, combining them into one system.
But obviously that got me wondering which other consoles or computers that share similar components could be combined, even if you couldn't use the same CPU and/or GPU for both? Anything a bit more modern than a Sega Master System?