Can anyone explain exactly what Dual Conversion means in regards to radio systems?

It's to do with the filtering of the radio signal dual conversion filters the signal twice and so is less likeley to get interferance than single conversion, as part of this dual conversion sets generaly also have a stronger signal

72MHZ is too high a frequency to accurately and cheaply amplify without adding too much noise. Instead, you subtract an alternate carrier band from the 72MHZ, leaving a lower frequency signal which has better signal to noise ratio. That is 1 conversion. Then, you can apply your tuning frequency accross the result leaving just the signal. That is the second conversion.

Have a look at the really good write-up at:
http://www.torreypinesgulls.org/Radios.htm

gus