Here's a picture from the
Zpacks Duplex page for the tent. With that roof incline sewn right to the bathtub floor, I think it is a sure guarantee that condensation will roll down inside and puddle at the bottom. If you are sitting out a rain storm, I am afraid that would be a big issue.

I've used a ZPacks backpack for the last few years (need to replace the last one, as the material delaminates after a few years). I liked the last one so much that I won't use any other brand.
The
Tarptent ProTrail seems to have a solution to that problem:

I would be tempted to try out the
HMG Echo II tarp cover by itself, leaving home the 1 lb bug shelter, and use a plastic garbage bag as a tarp under the sleeping bag.
