Using extra plywood as stringer for window boards?
If those stringers are unsupported by an underlying structure such as a substantial mullion, it would allow unacceptable flexion at the joint under high wind conditions IMO. Even with a solid wood stringer you might have trouble over that long horizontal span.

Might I suggest that you use your idea but with the plywood in a vertical orientation? I know this would double the number of sheets required to 4 (5 to join with plywood "stringers"), but it would be much stronger this way. Also, the smaller pieces would be much easier to handle and install, and easier to store when there is no hurricane threat. 5 sheets is still way cheaper than a new window that size, you could use the leftover for other smaller windows.