I think I'd be tempted to get them both checked by the vet for virus, any other cause and perhaps something to ease the soreness.
Do they both have access outside, as there's the chance they have both come into contact with some kind of cleaning/garden chemical. with the nice weather I know I'm starting to do things outside more. If there's a chance it could be an outside contaminant and their exploring territory isn't that big, I'd have a word with the neighbours to see if anyone has put anything down in the last few days, so you have something to work on if it happens again.
Hope they are both better soon.