Earlier I got a Dynex WS-007 RF modulator and I found it connected the left and right audio inputs together. After returning that RF modulator I found out from tech support that the "GE Video Converter RF Modulator with S-Video" "SKU: 23298" was also mono. They said that there is a patent on stereo output. Wikipedia also says that "Modulating a TV signal with stereo sound is relativly complex and most low cost home TV modulators produce a signal with MONO audio. Even some units that have two audio inputs simply combine the Left and Right audio channels into one mono audio signal."
So, are there any stereo NTSC RF modulators? I don't feel like contacting many companies' support departments because it's time consuming and they may not tell the truth. I also don't feel like buying and returning many products. I wonder what's the chance I get a useful response here.