Electronic – Do MOSFETs have voltage drop across source and drain when turned on

There are a couple of issues that you need to be aware of.

First of all, you need to make sure that they share the blocking voltage equally. Just like with diodes, you need to put high-value resistors in parallel with their drain and source terminals such that the current through the resistors is on the order of 10× the reverse leakage current of the transistors (or diodes). This will equalize the voltage drops to within 10% or so. Also, the speed of the individual devices might vary a bit, so it's also a good idea to put capacitors (typically 1 to 10 nF in line-frequency applications, less at higher frequencies) in parallel with them as well, to tame the transient behavior.

The second big issue is that the gate drive to each transistor needs to be relative to that transistor's source terminal; they can't share a common connection. Depending on the application, you might use transformers, optoisolators or specialized high-side driver ICs to allow each of the gate drive signals to float relative to all the others.

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.