I wasn't worried about the double inner panels flexing at all. I agree they aren't going anywhere once that baby is all glued up. I was simply trying to make my life a little bit easier by not having to worry about putting the two piece inner panels together. Horse a piece I suppose.
Either way, that's good advice on getting an extra sheet. Heck, anyone that is making a set of Alphas will likely be involved in another MDF intensive project in the future.
Speaking of which, have fun w/ the A/V-3's, they are such cute little speakers.