Moving one of the adjustor acts to rotate the arm around the center of the other adjustor. Since the lower balljoint is pretty close to being lined up with the front adjustor (in the left/right axis), moving the rear one moves the balljoint mostly forwards or backwards. Forwards gives you more caster, backwards gives you less. Since it's an arc, however, you'll also move the balljoint inwards or outwards by a little bit, which changes camber.
Moving the front adjustor, OTOH, will rotate it around the rear one. Since the rear one *isn't* lined up with the balljoint (in the fore/aft axis) it's at a different place on the arc , which means you get proportionally more fore/aft (caster) motion even though you're mainly interested in left/right (camber). Max negative camber means moving that adjustor all the way out, which is also going to be as far rearwards as the front adjustor can make it. So max camber is pretty much always going to be less caster than you could get if you didn't care about camber.
Of course, the factory adjustors also move the arms up and down slightly (because you're rotating the eccentric bolt inside a circle defined on the subframe), which is going to have the effect of lengthening or shortening the arm as measured in a plane parallel to the ground.
Double A-arms work well, but the downside is that everything is interconnected and changing any single arm length or pickup point will change every alignment spec by at least a small amount.
If you're ever replacing the bushings (and thus need to completely realign the car anyway), spend a few minutes turning the bolts and watching how stuff moves.