Well it's simple if you boil it down. If a drive shaft is perfectly straight from power to drive end then clearly you don't need a joint.
When it comes down and across like it does in real life, you need a joint - well actually you need two joints. One at either end. Well the problem is that UJs only work when they bend a little bit. Bend them a bit more and they're rubbish. That's why a CV joint is called a CV joint. Constant Velocity; they can bend loads with no speed variation. When you bend a UJ they sort of wobble or cycle fast slow fast slow fast slow. You need them in pairs and have to fit them so that they work opposite to each other and go slow, fast, slow, fast, slow. That's what they refer to as phasing. One crappy end cancels out the other crappy end. What a double cardan does is cancel itself all in one unit rather than have one joint at each end. BUT. This only works when the diff end of the shaft goes straight into the diff. As in - it goes in straight. If you get the axle castor right on the 80 then the front prop does go in straight. So castor correction and a double cardan or even a double double cardan aren't necessary.