The duel conversion receivers do use two intermediate frequencies, 10.7 MHz and 455 KHz. These were picked due to the fact that AM radios used 455 KHz, and FM radios used 10.7 MHz. The older single conversion used 455 KHz only.

Receivers used in RC started using the super-het concept in 1958 when the FCC allowed 6 frequencies that were for the most part 80 KHz apart. At that spacing, it was easy to filter out adjacent frequencies, so single conversion was used. It was not until the early 70's that EK came out with the first dual conversion for RC. EK was also the first to use the bridge amplifier in the servo that allowed two wire batteries, so they contributed quite a bit to the industry.

The super-het (hetrodyne is an ancient term for taking two signals of different frequencies and mixing them in a non-linear circuit) makes all modern receivers possible. This technology takes a fixed frequency from an oscillator, and mixes it with the signal from the antenna and sends the result to a filter that passes just the IF frequency (actually a limited band of frequencies get through). As people already stated, do it twice for dual conversion.

The real problems with receivers has more to do with the filter design, intermodulation distortion, and automatic gain control. The mixing works well if the power levels are controled.

The main advantage of dual conversion is that the image frequency of the first I.F. stage is now easier to filter out at the antenna, since it is now 21.4 MHz away from the pass band of the antenna. So a simple filter with a very low number of poles has an effective filtering effect. With the single conversion at 455 KHz, the image is only 910 KHz out. Note that with all 50 frequencies, that it no longer can keep out the possible frequencies that can be encountered at the average model flying field.

Yes, the JR is single conversion, but they don't operate on all 50 frequencies to avoid image problems.