Where are you going to find panniers that are 40 liters each?
My rear panniers (Ortlieb backrollers) are 40 liters for the pair. If I am carrying a lot of stuff, I also will add a 31 liter duffel on top, also made by Ortlieb. That is a total of a bit over 70 liters when I have the duffel stuffed full. (My front panniers add another 25 liters.)
I did not use my panniers as luggage on the flight, instead I had a fabric bag that was essentially a rectangular duffel bag that was shaped like a suitcase (olive green in the second photo). That fabric bag I left at the hostel I stayed at during the beginning and end of my trip.
My luggage for my Iceland trip in the second photo, the duffel that I had on top of the rear panniers was also my carryon bag. Handlebar bag was my "personal item" that I carried onto the plane. And I wore my helmet onto the plane so it did not get damaged. My bike was in the big black case, that is the S&S backpack case. (The bike frame splits into two halves so I can pack it in a smaller case.)
A luggage scale is the travelers best friend.
Will your bike be checked too? That only leaves one other bag to check if the limit is two. I would not dream of trying to take my 120 liter duffel bag on a bike, but that is pretty close to the upper limit for airline luggage sizes. If you can find a place at your destination to store something like that so you can retrieve it later, that may be what you need to do. Put your panniers and all your gear in it, but watching the weight limit.
https://www.rei.com/product/851365/p...le-duffel-120l
A military surplus sea bag might be another good option to put all your stuff into instead of a duffel like mine, it would certainly be cheaper. Worst case scenario is that you could leave it in the bottom of a pannier during the trip if there is no place to store it.
Some airlines have a carryon weight limit, some do not. I often put my densest stuff into the carryon for that reason.
A bike that is that heavily loaded in back does not handle that great. I suggest you try that as soon as practical so you can figure out if that plan is going to work for you.