Dan...
You can try this. I had to repair my Simms L2 boots near the end of last season and so far it’s holding up, but not sure how long it will last. Mine separated about a quarter of the sole by the heel.
I used Shoe Goo pretty liberally between the sole and boot bottom, pressed down and let it ooze out. Then I laid about an inch wide strip of window screening material I had over the oozing Shoe Goo on the seam between the sole and boot bottom. Smeared the excess Shoe Goo that oozed out all over the mess and let it dry with some weight in the boot.
So far so good, but like I said, not sure how long it’ll hold up.