The Dynex WS-007 RF modulator has separate left and right audio inputs. However, on the unit I got and returned they were connected together electrically, which means it's actually mono. According to the RF modulator article on Wikipedia "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." Dynex is certainly a low cost brand and I guess that the modulator was designed that way. Their online support told me that the RF modulator is stereo but I guess they were wrong.
It wouldn't be the first time Dynex produced products which don't work as they should. For example HardOCP reviewed a Dynex "400W" computer power supply which couldn't even output 300W. One of the two power supplies they were testing burned out at 300W and the other one shut down and wouldn't turn on for hours. At least in this case it doesn't seem like the item packaging or online description claims the RF modulator is stereo.