I know this is a dead thread, but google brought me here, so it lives as a zombie to the world. As such, I'm adding something which wasn't mentioned. I'm an engineer with some experience with waterproofing and extensive experience dealing with problems people create.
Everything in this thread is excellent and correct regarding where the two barriers are. Something that people don't realize is that, in most systems, that the tighter you make the air flow in the system core (between the barirers), the more you isolate potential problems. This can be very bad. An example is spray foam on the back of the exterior plywood. It will almost be a moisture barrier itself. Even if you have your ourter barriers and construction breathable, putting that in made it so that air and watervapor load will only be able to transport through the wood at some abysmal rate. A small leak will be visible from neither the interior nor exterior until the construction starts to disentigrate. This applies to any insulation you build tight enough; air does not flow through batt insulation at any appreciable rate (though liquid water may, which at least lets you know something is wrong earlier). In old construction, that wood got baked dry every winter. If you contain the moisture between the two barriers, you will eliminate a condensation problem, which is only really important because your house is insulated well enough so that you don't have zero relative humidity in the cold weather, but you run the risk of trapping the air/water. The air in the wall cavity has to be free enough that any moisture which makes it in there is intentionally lead back out through the exterior. One way to acheive this is to have appropriate air gaps. Here is an example:
Cathedral ceiling stating at interior = paint, taped & plastered GWB, vapor tight barrier, flush contact at joists/rafters and 1/4" minimum gap between insulation and back of board, battinsulation in the lower part of the joist/rafter depth, 4" ideal air gap over the top of the batt insulation separating it from the roof plywood (this is to prevent ice damming as much as anything else), pylwood/decking, watershedding roofing membrane, shingle roofing. [note that modern code may require more insulation than this will allow given the depth of the wood elements, but that the best way to address this is to install foil faced rigid insulation as your air barrier on the inside (foil inward), or you switch to all rigid insul on the outside of the roof, frequently i switch to metal roofing if i need rigid on the outside so as to avoid another layer of plywood... you can also leave the wood exposed if you go that route, depending on fire code.]
The same basic concept applies to walls... don't prevent air movement between those membranes! Air has to make it through your layers in the plane of those layers. Generally you have planes of insulation with intentionally allowed air movement on both faces of the insulation, but most particularly on the outer face. If you are using batt insulation and don't smash it up against the plywood, it should have adequate air paths for some evaporation on the back side of the plywood.
