I was watching a video about the Famicom and NES systems ad games, and it made me wonder something. It mentioned that if the NES expansion port could have been used, then some NES games, like the 1st Super Mario Bros. game, would have been in stereo. If I got a power adapter for my Twin Famicom, then an NES to Famicom pin adapter, could I play those game in stereo?

No. NES/Famicom are mono.

You can easily make a stereo split, there are separate outputs for pulse and tri/noise/DPCM channels. But this won't be real stereo, just random stereo separation, not intended by console and game developers.