Morning to you too.....(well it's 3:00am right now...so I guess you could say morning too...)
I'd say a min of once a month. Using bleach or anything that cleans well. Water bowl, I personally wash them every other day, and give them fresh water every day. Adding another hide in there would be good since you side that they are housed together. So if one does stress the other each one of them can have a hide if they want. (But most properly this wouldn't happen.....they like the temperature of one hide better)
Just a little note, keeping corn snakes together isn't that good. Two corn snakes together can stress one or the other or both of them into not eating and cause death later on. Two males can fight, one male and one female could lead to too early of breeding, and two females can stress each other out. Also canabilisum could occur. Just a little suggestion.
Good Luck and Happy Herping!