AMK Emergancy Bivy plus CS Apex?

I have never used an AMK bivy, but I use Apex a lot here in GA where humidity is the main killer. I've been able to go to sleep in wet clothes and wake up dry, and he quilt is dry too. I really like the Apex insulation and have taken the 5oz Apex down to below freezing with a Alpine Light jacket.

As for you suggested plan, my main concern is actually with the polycryo. I have heard that it is very difficult to sew, and I am not sure if the AMK bivy will be much easier. I haven't personally tried sewing with either materials, but others at BPL have used the polycryo and reported difficulty.

I actually don't plan on sewing anything. Instead I would use double sided tape that comes with the Polycryo (window insulation). I have yet to determine a way to hold the Apex in place using this method but I'll worry about that later ;)

What you are describing can easily be done with your existing down bag.
You use the AMK bivy as a Vapor Barrier Layer around your body inside the down bag.
Then use another AMK bivy as a waterproof outer bivy to protect from condensation.
The end result is that the down "sandwiched" in-between stays dry and lofty, and no moisture gets into it from inside or outside.

Now granted, this isn't waterproof at the head opening, and wouldn't be suitable for just laying out in the rain. But if you had enough of a tarp covering to keep the rain off the open end around your head, you'd be all set.