one compressor or two?

There is only one compressor, but two condensors.

My guess - one compressor or condenser and two coils or evaporators. Same on 06 - but I have to spend $10 a day to access the Toyota web site to find out for sure

Condensers do not drip water; evaperators do.

My bad.

More than likely one compressor on the engine, one condenser in front of the radiator, one receiver/dryer, and two evaporator coils and fans front and rear. And a lot of extra control gear that a normal car AC system would never need.

They won't be simple expansion orifices on the evaporator coils, or both cores would get cold at the same time even if you didn't want them to - the split thermostat controls prove that. And two evaporators on a common compressor would require a TXV for each core to provide individual control of the coil evaporation temperatures, or one would ice like crazy if it had a lower heat load on it - the compressor has to stay running if either evaporator thermostat is calling for cold.

I'll bet they have a 12V refrigerant solenoid valve in the liquid line ahead of each thermal expansion valve, and a 'voting system' where the compressor clutch kicks if either thermostat calls for cold. It's the only practical way to do it, positive control of the flow to each coil.

They might also have to add suction regulators on the "outlet" of the evap cores, so they can have separate expansion pressures. They do this in commercial refrigeration all the time, they run 5 or 6 different refrigerated display cases off one compressor.

The only way to use the "AC Amplifier" thermistor sensor at each end of the car is to kick the refrigerant supply solenoid coil off to prevent evaporator icing.

Though the compressor lockup / clutch cutoff circuit would still be valuable, they would have to split the two functions. If you cycle the compressor off to prevent the front core from icing, the people in the back seat can start baking cause you cut their air off, too...

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That was way over my head. I'll just take your word for it.

I'm thinking he was only looking for an explanation of the two puddles under the car (van) when he got home from getting groceries.

There is one compressor on the engine, and two evaporators (the part that drips water while/after the system is being/has been used).

Now the next question. Does it make affect fuel economy if the rear climate control system is used?

The answer is a definite maybe. AC affects fuel economy when the AC compressor is engaged, and the AC normally cycles on and off once the vehicle is cooled down.

When the vehicle is hot and being cooled down, the compressor will probably be engaged the whole time, but after the vehicle is cooled down, the compressor will engage as needed. If the rear AC is on, it may call for compressor operation while the front is not looking for compressor operation, but if it is off, then the front AC might need compressor operation more often.

The difference from using and not using the rear AC is probably negligible.

No. Well, anything that is a load affects fuel economy, but using the rear A/C is going to give much more comfort than it costs. The front A/C has to be on, so the fuel consumption will be impacted, but the addition of the rear is not a significant load. All that will be different is that the duty cycle of the compressor will be longer -- maybe.

It has to, at least a little bit. Work is work, and if the compressor is doing work, the energy has to come from somewhere.

But it will be the exact same amount of energy to run the rear air as the front, if you are removing the same amount of heat from the car. The only difference will be the electricity for running the second fan motor at the rear air system, and that's negligible.

And the 1/4 MPG (or less) hit on the gas mileage to run the AC is more than offset by arriving at your destination cool and fresh, rather than hot and sweaty and ready to kill something.

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