A dual gate MOSFET is a FET with 2 gates
In the work that I do .... RF field, dual gate MOSFETs are often used as a mixer device in a receiver or transmitter circuit.
for example ... in a receiver, the incoming signal goes into one gate and the local oscillator goes into the other gate and mix within the FET to give an IF output signal which would normally be the received freq minus the LO frequency.....
OK here's a sample circuit with the receiver frontend amplifier and mixer transistors
The 3SK73 -- arrowed -- is the dual gate MOSFET
cheers
Dave