So I tried to make an FM stereo encoder. You can see my schematic in the attached picture. I do not have a scope with me right now, but that will probably be my next thing to try if I don't get any hints here.
When listening to it plugged into a transmitter, the problem is that it seems to separate the channels, but they flip back and forth between left and right. It seems like it may be something with the 19khz not being in phase with the 38khz maybe. Another possibility is that FM stereo is DSBSC, so does that I mean I have to notch out the 38khz? I just tried and it didn't seem to make a difference.