Double up computer PSU for +24VDC @ 15A?

I recently bought a high-power 6" blower fan that needs +24VDC @ 15 Amps. I was thinking about a cost-effective way to generate this power, and I realized I have several old 400W computer power supplies laying around. Two are identical - they each have a single +12V rail rated for 22 amps. Here is my quesion: Can I link two PSU's in series to generate the required 24V? (Can I connect +12V from PSU1 to GND on PSU2, then power the blower from PSU1 GND to PSU2 +12V?) I am not very familiar with the inner-workings of the computer PSU, so if this is impossible for ____ reason (it will short out, etc) please let me know!
Thanks.

The GND (black) wires of each supply are normally connected to the frame of the respective supply, and in turn to earth (green USA, green/yellow EU) wire in the power cord. This makes GND of every supply actually earth potential! Connecting the GND of one supply to the +12 of another would cause a short!

You could probably modify one of them to "float" the black 'GND' but this would be dangerous if you don't know what you're doing.

Also, are you aware of the start-up requirements? (Minimum loading on 5V, and power-on command on one of the wires..)

I have read somewhere that somehow linking the +12V and -12V would produce 24V.

-Chris P

Yes, using the +12v as + and -12v as GND would give you a 24v potential, however Im not sure how that would work with the whole current issue.

Yeah I just looked at a (small) ATX PS I have and its -12V is only rated for o.5A. However, there must be a relatively simple circuit for converting a 12V line to a 24V line. Some time ago I saw a diagram for a 5V -> 24V converter.