The easiest way to do what you want would be to read the ADC value into a variable using the read_adc_voltage() function and write the variable to the DAC using the set_dac_voltage() function.
The code below reads a value from the ADC channel 1 and writes it to the DAC channel 1.
Main program function
adc = ExpanderPi.ADC() # create an instance of the ADC
dac = ExpanderPi.DAC(2) # create an instance of the DAC with the gain set to 2 so the maximum voltage is 4.095V
# set the ADC reference voltage. this should be set to the exact voltage
print("Updating DAC from ADC")
# read the adc voltage on channel 1 to a variable
involt = adc.read_adc_voltage(1, 0)
# write the variable to the dac on channel 1
if __name__ == "__main__":
The ADC voltage reference is set to a value of 4.096 which should match the voltage of the reference on the Expander Pi. The DAC is set to use a gain of 2 which gives it a voltage range of 0 to 4.095V. This allows the DAC to have the same voltage range as the ADC so any voltage between 0 and 4.095V on the ADC input will be sent to the DAC output.
A variable called involt is used to store the value from the ADC before writing it to the DAC.
I hope this helps you with your problem.