I bathe one on Sunday and the other on Monday, two are too much in one day. Which ever is the dirtiest, usually Artie first on Sunday and Abigail on Monday whether she's dirty or not. Just so she can get used to it.
If it's raining or his coat looks dry? I spray him with a leave in Conditioner, Mink Oil so he won't get matted in between baths. I haven't had any skin problems with either one, so once a week seems to be fine?