What parts are in each bag for the Star Wars Death Star 75159?
I have an idea that might just work for you. 
You clearly have two of everything that you need, so I recommend use Bricklink and go through your list selecting the quantity of bricks required from the pile of bricks that have been opened. That should leave you a pile that you do not need (for the second build)
This will be a good starting point.
Good luck.