I’d probably cull the older, fatter one but personally I’d give the younger one another chance. I’d make sure she didn’t get too fat, and of course try to keep stress to a minimum next time. (So sorry to hear about the dog incident, and I know it’s not always possible to avoid such stresses.)
I’d also make sure the ewes have plenty of minerals throughout pregnancy, probably with your breed being prone to ‘doing well’, using a good chelated drench such as Ovithrive rather than a lick or feed. Being in tip-top condition (without being fat) should help them avoid prolapsing if there is another event another time.
If you do keep one or both on, you will of course be alert for any signs of prolapse and will fit a harness or spoon at the first sign, should there be a recurrence. So really you’ve little to lose giving the younger one a second chance - but I know others on here have had different outcomes.
As well as keeping them slim, and giving a good chelated drench, also avoid them eating too much forage at a sitting in the later stages of pregnancy. It might be better to put out a bit of hay three or even four times a day than feed ad-lib (as long as they’re getting enough, of course) to stop them overfilling their bellies at one sitting.
Hope all goes well with the lambings this time, and whatever you decide about keeping or culling those two, know that you made the decisions with the ewes’ best interests at heart, and feel good about that.
I don’t suppose you know who owns the dog? And could ask them to pay for replacement ewes as their dog caused the prolapses?