There's no rules on either site that forbid it. I know there are some people who frequent both sites. Some might ask why you posted different versions on different sights, though I don't think anyone would make a big stink over it. DD is more active, but ABDL Story Forum the reviews/ comments are generally more in-depth. (Also, note- critiques are allowed and encouraged over there. Not saying people will be mean, but they do point out problems with stories- broken verisimilitude, flat characters, actions/ elements that make no sense, plot holes, giant wall o text, etc. It's great for writers seeking to improve their craft. Not so great for people who just want ass-pats and get mega butthurt when it isn't 100% praise.)