I just checked your older posts and I remember when you were first rescuing these two, AKA Farmer and Dell.
I reread your first post and you said that Farmer (did you rename?) is a marker. I would also like to suggest a belly band for this little guy. Marking is different than peeing since it's a territory issue and not that he doesn't want to pee outside. Get his some belly bands and only take them off when you take in outside to potty. There are several people on YT that sell them. They wrap around the waist and I think you put a pantyliner inside so they cannot mark. This could be an issue that you might never resolve since marking is different that not being housebroken.
He just might have to wear a belly band all the time while in the house but I think you can get it under control.