Back-to-back MOSFETs: common source vs comon drain?
If you need to drive both MOSFETs from a common signal you have to tie the sources together or the body diodes will stop you turning them off.  Every MOSFET has a diode in parallel with the drain and source electrodes.



The gate drive needs to either have a floating source applied between the common source and the common gate.  Or have enough swing to guarantee enough bias for the entire swing of the input signal.  The max Vgs will often prohibit that approach.