Whats the overrall power available from two PSUs in series?
The maximum current you can draw is the value of the lowest one.

In this case the 100W supply could provide 8.3A, while the 200W supply could provide twice that at 16.6Amps.  If you attempted to extract more than 8.3A the lower capability power supply would collapse (and possibly damage it). So the output is limited by the smaller supply to 8.3A @24V, 200W.

If you put supplies in series it is often desirable to put a diode across each supply so that under overload conditions the output polarity does not reverse.  The diode should be able to pass the current of the highest output supply.  The diode polarity should be such that under normal conditions it is reverse biased.

A similar arrangement is used with photovoltaic solar arrays to avoid reverse biasing modules if one gets shaded.