Thanks for all the input!
First off, yes, I am firmly aware of the biner-over-edge issue. Not only am I aware of the danger, but after building scores of trad anchors I have come to develop a feel for the movement of anchors and how a biner can be loaded over an edge even though at first it seemed that it won't. I'm not saying I'm a master

, just that I've seen this danger in action. In this case all three biners were hanging straight down a smooth, blank, vertical face.
OK, I'll definitely post a pic as soon as I'm back. For now, a sketch of my setup (nowhere near to scale:
Attachment:
File comment: Black circles are cam-side biners, red is rope-side biner
slidingX.JPG [ 4.33 KiB | Viewed 630 times ]
Now, most of you are very experienced and I would like to learn something from this. So if anything I say here seems arrogant or wise-ass, I just got my wording wrong. I really appreciate the criticism here.
I'll try to list the concerns mentioned about two pieces equalised with one sling, and my current reasoning about it.
1. Rope biner will pop off if one piece fails.
A: Not with the X setup. If one piece fails, the rope biner will slip from roughly the middle of the sling to the other end. Will be clear with pic.
2. Remaining piece will be shock-loaded if one piece fails.
A: Not so badly because the rope is dynamic. I've read many forum discussions on the sliding X, and many tests showed that it's by far the best equalizer for two pieces, in terms of load distribution. Also the test showed that with dynamic rope the shock load is a myth (will try and dig up my references for this).
3. Upon loading, the two pieces will be pulled to each other.
A: This will also happen if you place two pieces and clip them on seperate slings (saw this with my own eyes). I believe the only way to counter this, is to
(a) keep the angle between the pieces at the rope-side biner(s) as low as possible by making placements as close to each other as possible (in my case it was roughly < 25°)
(b) ensure the pieces will handle load in the rope-side biner's direction.
(c) ensure that if one piece fails, the change in direction in the other piece will be so low that it will not cause failure (if you see to (a) this should be OK).
shorti, if I understood correctly, you just described the American Death Triangle, which is a HUGE no-no.
http://en.wikipedia.org/wiki/American_death_triangle*EDIT* My bad, you were warning AGAINST it

Sorry!!
The one drawback I do have, is that if my slings fails, the whole placement is nullified. However you can say the same about a normal quickdraw/runner and in this case the sling was hanging down a smooth face - no prob as long as the sling is in tip-top shape.
That's about it from my side *assumes fetal position and waits to be ripped apart*