After 90 minutes you pretty much have used up the Alpha that will turn to Iso-AA. After 90 minutes the ISO-AA degrades. IIRC some of that is pretty harsh tasting.
Some of the recent research has stated that it is not the sugar concentration that inhibits the utilization, but the break material. More grain = more break material, so the models (Tinseth, Gaeretz, etc) still hold. If you boil and leave the break behind, could that give more utilization in the second boil, as it would have less break material?
I have though of making a beer where the wort is boiled with no hops, chilled, and transfered to second kettle. Then boiled again with the hops added to see if the utilization or bitterness would be higher. Due to the fact that I am retired, I just can't seem to find the
time motivation to do that lengthy process.