There are a couple of concerns with the turret, I think:
1) does the turret floor on the upper floor have sufficient strength to take the shock of firing?
2) Does the arrangement of your upper and lower floor guns limit elevation on the upper floor guns?
1) no idea I assume you could make it sufficiently strong as you are starting from clean paper.
2) Mark 5 had -7 / +20 degrees (both top 12' and bottom 8' ) and that's fine till you get good directors post WW1.
(from navweps, Note: It should be mentioned that this maximum range was of little use at the time these ships were built. Fire control systems and rangefinders capable of accurately firing at ranges over 10,000 yards (9,140 m) were nonexistent.)