Ed,
This doesn't make sense to me.
"With the back plow you'll be carrying a second load across the lot and scraping cleaner(hydraulic down pressure)as you go ...requiring less de-icing material."
Two things come to mind.
1. Doing a lot, the front blade would scrape clean, or almost clean.
2. With the rear plow down, you'd leave a "trail off" trail, or even 2 trails if you kept the rear plow in the straight position.
It sounds like using a rear plow on a lot with a front plow, would make a mess. Having never used the 2 together, I can't say for sure, just a feeling I have. Not only that, but you'd wear out 2 cutting edges at the same time.
I have a feeling that the "load" in front of the rear plow would be lost to trail off on every pass. Now a set of wings on a rear plow, well, that sounds like it would work for doing lots. At least the wings would help stop trail off.
~Chuck