Connecting two PSU's in series
It would depend on the type of PSU used, linear PSU's normally use a regulator of some description at the output stage and can sink current (but could get damaged over time) as PSU1 would have to do this. Most switch mode PSU's have a capacitor and diode. These types don't really care as the diode would prevent the current from PSU2 from going where it should not.

I have done this a lot with LAB grade PSU's and had no problem as well as some high current switch mode power supplies (30A) and had no problems, but our power here comes with separate earth as we have Live, Neutral and Earth. As long as your mains side is earthed to your chassis and the output is isolated from the input (most PSU's have a transformer), I can't see any problems other than stated above.