Yes, 1/2" cordierite will char slightly better than 3/4" fibrament. Both pale in comparison to soapstone and 1" cordierite, though. As far as the cordierite pampered chef stones breaking on you... transferring hot stones causes substantial thermal stress (and is best to be avoided), so that probably was the culprit, but I still expect a little more longevity from cordierite than that.
As I mentioned before, the separate broiler setup definitely complicates things. You really don't want to be moving a blazingly hot 40ish pound soapstone slab around. We've been dealing with single direction heating issues in the converted BBQ/Egg threads, and, it's extremely difficult to get the right dome/hearth heat ratio. With the heat coming from beneath, the hearth is invariably too hot and you don't get proper top browning. Out of every possible scenario, I think your best bet, although most expensive, would be two soapstone slabs. Even that doesn't guarantee you the proper ratio, though. I think, though, that if the gap between the stones is small and if you preheat the stones and then turn the heat off for a bit for the heat to travel from the bottom to the top, you should be alright.
Another possibility- and this gets pretty experimental, is a steel slab ceiling. Not a steel pan, something that can really hold a lot of heat (1/2" thick territory). In theory, a highly conductive steel slab should be able to radiate heat down towards the pie better than soapstone. You might experience some slight recovery issues, because the high conductivity will cause the steel to lose heat quickly, but, that same conductivity should allow it to come back up to temp in a short amount of time with the door closed/heat cranked.
That's a lot of theory, there, though. People have been using iron pans for hearths for ages, with, if they understand the material, good results, but I don't know anyone that's gone with a metal slab ceiling. For all of the top heat handicapped, separate broiler, gas oven owners out there, though, this could be a viable solution.
Lastly, as I was typing this, one other potential top heat solution came to me. It's probably the least elegant of the bunch, but it's the cheapest. In a single soapstone slab scenario, you're going to want the stone on the highest shelf possible to make use of the heat radiating off the top of the oven. That scenario will give you an undercooked top, but... you could start the pie in this position until you get a good char on the bottom and then transfer it to the shelf directly below the stone to brown the top, in effect 'doming' the pie against the bottom of the stone. The closer you get the pie to the stone, the better- if the position of the shelf gives you too much of a gap, use a metal peel and hold it in place.
Oh, and stacking stones- I think stacking is fine, although I think it's generally a good idea to match the dimensions of the bottom and the top layers so everything can heat evenly. Even heat makes for both even cooking and happier stones.
