The "cow teat" will reduce to normal size if the milk is taken, but you will probably have to milk some milk off by hand a few times a day for a day or two, until the teat is less engorged, and then each time when you've made the teat smaller, get one of the lambs onto that teat if you can. The sooner they start using both teats, the sooner the big one will stop getting so engorged.
By the sound of it, they are both managing to get plenty from the other teat at the moment, so are not hungry enough to take a bottle or to take the large teat. You could pen them together but so that the lambs can't get to the udder, for a few hours until the lambs are hungry, and then milk some off the big teat so that it isn't to engorged and then get one lamb onto that teat to have a good drink before you let the other lamb onto the normal teat.
I had a favourite Shetland ewe used to do this. I'd usually get one of the lambs to be using the "cow teat" without my help by Day 3 at the latest. She was very tame so I could just do this in the field, didn't need to pen them.
Don't keep any offspring for breeding, this is hereditary. And if you don't want to do this every year, mark the ewe and cull her this backend.
Oh - and now is a good time to milk off some colostrum and pop it in the freezer, in case you need some later