I agree that it sounds as if it would leak, but that's code, IRC, ARMA,
CABO, etc. One layer on 4:12 and grater, two layers fastened together on
lesser slopes. The deck is protected by the underlauyment, the edge by the
drip edge, the and two layers of shingles above that. My guess is, if it
leaks there is just gets into the overhang. Anyway, no one uses this method
any more, too messy and much more time consuming. But it's still code. Can't
say if your locals allow it or not, you could ask if you get a chance (not
that you would ever want to do it of course.)

None of the cities I work in, would allow two layers cemented together,
for
a substitute. How would two layers stop ice back-up? It would just
back-up
under the layer closest to the sheathing, and/or back-up on top of the
layer
closest to the shingles, which is perforated by the nail shanks. Granite,
it wouldn't back up between the layers. But, the damage would be the same
if
there wasn't any underlayment.

As a licensed & registered GC in the cities, I'm unfamiliar with State
Licensing which is required for plumbing/HVAC/electrical.

I was licensed by the Ohio Board of Building Standards in Building and
Mechanical (all they required) as an out-of-state, prefab/modular inspector.

I have to take something back, when I said I "never heard of two layers
cemented together", I was mistaken. It seems to me, about 30+ years ago, I
recall troweling Karnak 19 to adhere two layers together. Had I had a clue
back then, I would've questioned how that would stop ice from backing
underneath the top/btm side of the two layers.

(Walk quietly and carry a big mop!) g
Best
Dennis