Q:

Clustering the 3.3v and 5v rails

Is it possible or advisable to connect the 3.3v or 5v rails of each Pi in a Pi cluster to other Pi's respective rails, so as to minimize loads across any one particular Pi's voltage rail? If it is possible, then it should also help with heat disapation over the combined surface area of the components comprising each voltage rail for a given load. That might potentually lengthening the life span, or offer the ability to provide more power than any one Pi could on its own. To be clear, I'm not talking about sourcing more power from or to any GPIO pins.
I am speculating that the main concern might be tiny differences between components on different Pis might cause currents to be sourced from from just one Pi's rail rather than splitting the load across each. 
Additionally, I am aware that there are difference in the components that makeup the power supplies between Raspberry Pi models; and that if one were to try this, they shouln't connect the rails from different Raspberry Pi models.
I ask this question only to give me ideas on possible aplications rather then to apply to any existing aplication.

A:

NO!  Do not interconnect those power sources or you will release the power supply Genie.  Connect one ground (GND) line between the two Raspberry Pi's and any other power supplies all you want but never the power sources.
The 5-Volt is the power coming from the USB connected power supply.  The 3.3-Volt line is a "regulated" power conditioned from the 5-Volt source by your Raspberry Pi.  Power regulation tries to force 3.3-Volt to stay at that level even when the current load varies.  It uses an internal reference such as a zener-diode which might be slightly different between the two Pi's.  Interconnected regulators would struggle against each other for control.

