For example, realm of fire artifact with cold ferocity on old blood on carno. both say an unmodified hit roll of 6 makes 2 attacks instead of 1. I looked in ghb and core rules and saw nothing about this.
yes. there is no limit on how many abilities proc off of a dice roll unless specified in the text of the ability itself if you have 2 sources of exploding 6's, each hit would produce 3. if you also had mortal wounds in addition on 6's you would get 1 mortal wound and 3 hits