I agree that a large enough tarp doesn't require a bivy, but I still bring some kind of bug net for bug season.
I often find that the bugs can be intolerable where I hike and so will use a Meteor Bivy(half bug net) under m tarp.
I also find that if it doesn't look like rain, I'll sleep only in the Meteor and then when it does rain in the middle of the night, deal with getting the upper half of my bag wet or get up and throw the tarp over me.
Many cases that I wished I could just zip up for full coverage in a light rain, even though I know some water would get through the DWR.
I'm only speculating on the double top layer idea, but thought people might have some ideas.