I am using an OnSemi NTGD3148N dual MOSFET in a project. I cannot find a specification anywhere in the datasheet stating the isolation between the devices; and this holds true for most dual MOSFETs for example Si4946EY (which I used in a different project.)
Obviously I can't put 100kV on one drain pin and 0V on another without something blowing up. (Ignoring for the moment that it can't take 100kV D-S; the source in this example is left floating.)
This is important for me, as I am switching two independent loads which cannot be allowed to short together. E.g. one might be 12V and another 5V.