The difference between HBT2 and HBT1.

HBT2 operates a different software program with redesigned circuitry
HBT2 delivers speed of less than 20ms
HBT2 is more responsive
HBT2 is more accurate with less than +/- 1Hz
HBT2 supports more drone frequencies
HBT2 has an inbuilt speaker for the Metronome

As the accuracy of HBT2 is higher than HBT1, the two tuners are not compatible.