First, OP, excellent question--and excellent responses.
Second, I will ask the stupid question: are you better off with one anchor most of the time even with the set up in the diagram?
Third, in the diagram, the secondary rode is optionally longer than the primary (to the connection point). Does that mean you do not necessarily want to pull equally on both anchors, and if so, why?
One anchor is generally best.
* Two anchors cause the boat to swing differently than other boats, increasing the odds of a bump in a crowded harbor.
* More likely to have some drag down on you, since your rodes are spread.
* More complicated if you need to leave in a hurry. Really complicated if someone drags and fouls your rode.
* More complicated to set. Power setting each individual anchor also takes time.
* Will tangle if you rotate enough times.
* May increase the stress on each anchor (not equal loads).
* Rodes may trip anchors.
Longer leg. Look closely at the illustration; one is pivoting fluke and one is roll bar.
This is actually a very specific, through rather common case. The most common reason to use two anchors is super soft mud. Otherwise, one anchor is enough and is a better choice. The most common secondary anchors in mud areas are a Fortress or Danforth, because they are so good in mud. However, they are not very good with direction changes. Thus, you set the Fortress farther away, expecting that the smaller general purpose anchor (Rocna, Claw, Delta, etc.) will drag a little, but using its ability to drag stably and reset to buffer the direction of pull on the Fortress, which is at its best when the pull is in a steady direction. In this way, the two anchors are synergistic; the GP anchor adds stability and the pivoting fluke anchor provides the real holding power.
Differing leg length also reduces the chance of the anchors fouling each other. A minor likelihood.
The only reason you would use a symmetrical layout is if both anchors have the same holding capacity and characteristics, which is unlikely. One will always be bigger, and the bigger anchor should be farther away. This is a little like tug of war; you put the big guy at the end of the rope.