This is a network wide feature, meant to allow the same user to provide two different, perhaps contradictory or incompatible options to the same question. It's not a particularly useful for the main History.SE site, because most of our questions deal with historical facts. Answers should strive to provide a full answer to a question, so if there are two explanations for a historical event, a good answer should incorporate both of them.
In contrast, on some other sites - including our meta site - question can be more subjective or be solved in multiple ways. For example, suppose we have a meta question on what to do with the tags united-states and united-states-of-america. I could post one answer advocating that we merge the latter into the former, and another advocating merging the former into the latter.
Both are valid options, so by posting both, the community is able to vote on which solution we prefer. It basically means we don't have to wait for someone else to come along and champion an opposing view.