How to connect unused MOSFET from Dual N-channel MOSFET?
Connect unused gate and source to 0 volts or
Connect source to 0 volts and gate to 0 volts via a 100 kohm resistor and
Leave drain unconnected

Instead of 0 volts, if it's inconvenient connect source to the same potential as the other source and gate the same or via the 100 kohm resistor.