Hi & welcome to AA!
I have not build a plywood tank, but I have seen tanks with multiple windows. So basically, you would have a vertical frame member to tie the 2 pieces of glass together, giving you 2 separate glass windows in the front.
I wouldn't just silicone 2 - 5 foot pieces of glass on edge & use that as a single 10 foot pane. FAIK
, no one had done it like that before. I wouldn't want to be a pioneer in new tank construction with that much water! <Older tank bottoms with 2 piece construction always reinforce the seam with a hefty piece of glass on top ... They must have a reason to not trust that to a simple side to side butt joint.>