Duel conversion describes how a receiver deals with the signal. The transmitter does not know or care about single/duel conversion. The receiver does care that it has the proper crystal. Single and duel conversion crystals are not interchangeable. Put a single conversion crystal in a single conversion receiver. Duel crystal in a duel receiver.
Oh, I said above that I was ignoring PCM/PPM but I will mention it anyway for completeness. Don't use a PPM receiver with a transmitter that is on PCM and vice versa.

Want a more technical description? Look [link=http://www.mp.ttu.ee/risto/rc/electronics/radio/signal.htm]here[/link]