I didn't double baffle mine 1299's. I think I read somewhere on here that Erich said it wasn't necessary. I did however use window bracing inside the cabinet. I made sure there was a brace on the side of each woofer in the cabinets. After the braces, the baffle seemed pretty stiff to me. I think most 1299 builds on the forum here have used window bracing if you need to see some examples.
If you do double baffle, you may need put a notch in the baffle for the speaker terminal on the mids. Those were pretty tight and I needed to tilt them a bit to get the terminals past the baffle when placing them. I don't think they would fit a double baffle without a notch in the baffle.
Since I did not double baffle, I glued some small blocks on the back of the baffle for some extra thickness when mounting the woofers. Unfortunately, I don't think I put pictures of that in my build thread.
Best of Luck!!