You may need to put an "X" brace in the back/sides to stiffing the table along both dimensions, as there is no piece running both vertically and horizontally on the table. Might end up leaning.......not that I'm speaking from experience or anything.....LOL.
You might try to configure one of the sides to be big enough for an XL......ya just never know what the furture may hold. Or Craig's list offers......
Double check the 21 inch opening vs. the 16 inch height to make sure the opening is big enough, the egg's felt line height is where you want it and there is room for the spring mechanism above the table. Common mistake is forgetting to take into account how the egg will sit, using the egg feet with or without stone pavers and such.
If it is a solid surface table you might be ok. If it's a planked top you may have trouble supporting boards behind and along side the two openings. Just look at the table and think how or where will I nail the board ends.
On my corian top I put an OSB (roof decking) sheet underneath to give the Corian some support and rigidity. Might do that for the granite section. Could get tricky though dealing with moisture, rain.
Your table plan is very similar to my original set up, picks above. I'd bet a cold beer after a couple cooks you'll be looking for ways to find more work surface. The middle is 32" wide, or 16" per egg, one pan per egg and the space is gone. The table ends (end to 21 opening) are 11" inches wide, not really wide enough to do much with, except set tongs or dangle a pan on edge. Dangles get knocked off, not that I'm speaking from experience or anything. That is why I added the folding side shelves.....12 inch side shelves on each end increases the 11 inches to 23 inches and that is more room than you have in the middle (32 vs. 2 @ 23).
That's all I got on one cup of coffee.....good luck with the project. T