There have been a few different synthesized frequency type transmtters... They don't catch on for "the most implausable" reason... they cost a little more.

When you compare the extra cost of the synthesized frequency transmitter to the cost of buying 2 extra frequency modules for a TX with that capability... you come out ahead with the synthesizer. (and its a heck of a lot quicker than sending in the TX to have it tuned to a new frequency if you want to be legal about changing crystals...)

Fortunately the technology fr the synthesized frequency system is getting cheaper... so we may soon see some 4 ch radios that have the capability.

But that creates another problem... You have to remember which frequency you "tuned into" on the synthesizer... so you can use the correct frequency pin.