Super idea - replace one tunnel .......... with two. Probably not. Tunnel slap is definitely normal. I have only seen/worked on one cat where tunnel slap would be considered abnormal - the tunnel was built high enough to stop it occurring in all bar the absolute worst conditions but when your rig is 90' long that is possibly a bit easier to do. In the meantime I found an easier way - buy a mono.
For the extra 5 or 6 knots, I guess I will just have to put up with the peace and quiet. Oh well. When it gets a bit choppy "George" usually drives so I can sit back and relax. We just take our time and plug away.
Try Luke from Dolphin Marine I got my 5.3 Whittley on a trailer & 2 stroke 90 Mercury for $19,000