Are dual MOSFET devices isolated from each other? If so, to what degree?
If isolation isn't specified in the datasheet then I wouldn't assume it's there.  Most likely both FETs are on the same substrate.  That means they are sortof dual diode isolated, but the breakdown voltages of those diodes probably isn't much more than the max D-S voltage allowed.

In any case, that's just guessing on my part.  If it's not in the datasheet, then its not in the device for practical purposes.