I'll explain where I'm coming from....I've been rigging concerts, singage, and almost everything else that's come into work at the stampede grounds here in Calgary for the last 6 years. I'm also a rope access/rescue technician. I have been studying the effects and physics of pulley systems for the last 3 years.
In the warn example where the vehicles are about 90* from each other, the rigging on either the stuck vehicle is not as important as where the pully block (change of direction) is. If the angle is 90*, then the forces on the block/pulley are 1.41 times the force on the cable. This is due to the physics on the rigging. So...lets say that the winch is pulling at 6500lbs, then the force on your pulley block is almost 9200lbs. Please be cautious with these kind of recoveries as this is where equipment will fail!
If (for example) you're going to use your winch to pull something into a tree to hang it (I dunno....maybe a moose). The force on the pulley and branch in the tree will actually be double whatever the weight is what's hanging. So...the moose weighs 300lbs, you hook your winch up, pass the cable up through the pulley and back down to the moose and start winching. The winch pulls with 300lbs force (pulling down), and the moose weighs 300lbs (also pulling down), hence the doubling of weight and the pulley gets 600lbs of weight hanging on it.
The angle between where winch (or whatever is doing the pulling) and the load is critical. At 0* (like pulling a moose up into a tree) the load "on the pulley" is double whatever the weight is of what is being lifted. At 90* it's 1.41x, and obviously when the angle is 180* (or the line is not re-directed) the load is whatever is being pulled.
For a two-to-one advantage system, you're very right. The pulley has to be attached to the load, and is moving. In the above example where someone attached the pulley to a tree, and ran the winch cable to the tree and back to the vehicle, this would be a 2:1. The tree has to be treated like the moving object here...(I know, it's not moving). The reason I say it is like this, is due to the fact that your winch is not moving in relation to where the cable is connected back to the vehicle. Also, for every 1' of cable that is spooled back onto the winch, you only move 6". ie...2:1.
I do hope that I don't come across the wrong way, but I have seen people get the wrong impression when it comes to pulley systems. If you want something to get confused over, when pulley systems get into compound and...even better, complex pulley systems. I don't mean to offend, but I do hope that it clarifies a little....