They can be staked to increase efficiency but this doesn't increase cooling capacity.
A normal psu can be used but this requires opening it up and joining both 12v rails to get the desired amperage and a relatively beefy psu is needed.
As modern processors are producing more heat direct die cooling is becoming less viable due the tecs running close to their limit as once they their limit they burn up and will more than likely take the cpu with them due to the extreme of temperature. This is where chilled water is taking place, by using the tecs to cool a water loop sub zero temperatures while using another water loop to cool the tecs you can achieve temps at full load of a couple of degrees below zero.
I will be playing with a chilled water setup at some point in the future once i've got a rig to replace the eeepc, but i'm hoping to fit this into a relatively small case compared to whats used. The cm 690 II to be precise as this has the capacity to allow for twin dual rads.