@Ryan @Ryan agree with Ryan here completely. There is a possibility to raise LT1 power and not LT2 power, so I would say they would NOT be coupled in all situations.
You may see an increase in LT1 power…and an increase in time to exhaustion at LT2 at a power, but not always an increase in LT2 power in linear fashion with LT1.
I ride brevets which are the non competitive version of ultra distance cycling. One such event is Paris Brest Paris (PBP) 1200km / 750 miles in a little under 4 days elapsed. For most riders they will be on the bike moving for at least 50 hours of that time. Try riding 12.5 hours a day, 4 days in a row to get a feel.
We know that drag increases with square of speed, and power demand with cube speed. It stands to reason that energy wise the most economical use of your available energy stores is to go as slow as possible. But there’s a minimum speed you need to meet, and if mortal you need some sleep (some riders don’t sleep at all over this distance).
Digestion is also an issue for many. What works over 6 hours of riding doesn’t necessarily scale to 50 hours of riding.
Combining sustained energy demand and digestion issues. Minimising the amount you need to eat is one strategy. To achieve this you’re trying to rely on your fat as much as possible. This means riding at or below LT1 as much as possible.
From the above raising your LT1 as high as possible is key to a faster overall pace in ultra distance. That faster pace allows more time for stops, more time for sleep, more contingency. You can’t replenish all your carbohydrate stores on the bike. You need stopped time to eat real food. Just before sleep is perfect, to replenish all the carbohydrates you’ve burnt through over many hours.
If I’ve neglected the long training rides then it shows in how much my brevets turn into type 2 fun and a real battle. That’s regardless of how much high intensity work I’ve done.