Hi Eric, it sounds like it could be a routing/selection problem.
First, are you using Simple Instrument Tracks or split audio/Midi. If SIT, try converting them to split and try again.
If they're split, what happens if you solo just
one synth, select BOTH tracks and export again. What do you get?
Routing: are all of your audio tracks (and the synth audio tracks) being output to your master bus? (Tip: mute your master bus and anything routed to your main outs will be audible (and won't get exported via "entire mix"))