I couldn’t find anything explicitly definitive either, though I did find this
which says that your innate retal DOT does not stack with retal DOT on skills with %retal damage added to them. No mention of whether retal DOTs on multiple skills stack with each other, but I strongly suspect retal is like weapon damage in that there is one unified DOT no matter how many skills it’s attached to. @Ceno, can you confirm or deny? (It was your answer in the thread.)
Edit: my wording here is confusing. See Ceno’s post below for a clear explanation of retal DOT.