I have a question that is relevant on StackOverflow and also here. I reposted the question here in this forum with a reference back to StackOverflow.
Right now, there's no way to migrate a question from one site to the other. So, personally, in situations like this where both communities can contribute to and benefit from the subsequent question, I'd encourage cross-posting.
The risk you run, though, is that the question is answered well on one site but not the other. I wouldn't want anyone to manually copy-paste an answer from one site to the other (this would also result in someone illegitimately gaining reputation points by plagiarizing an answer).
The trick with your question, though, is that it's not tagged well enough to live on both sites. Your post on WP Answers looks fine ... but your post on SO isn't even tagged "WordPress" ... so it looks like a more generic e-commerce question than a WP question.
Unless I misunderstood it, your question is specific to WordPress plugins, as you did not ask about other e-commerce platforms. In that case, I think it only belongs on this site. Of course, since Stack Overflow is currently a magnitude larger than WordPress Answers, I understand you posted on both sites and also got a good reply on Stack Overflow.
The linking is essential (maybe also add a link on the SO version), so others can find all the information even if they only visited one site. Also, if you accept an answer on one site, encourage the user to also post it on the other site so that question gets an answer too.
Similar questions have been asked on the main Stack Overflow Meta site, and the consensus there seems to be that it's OK, but try to keep the different audiences in mind. So if your question was "I want to create an e-commerce website and the choice is between Drupal, WordPress and Joomla", that would not be a good WPA question, but it might be good of Stack Overflow. Sometimes you can ask a general question on one site, then a specific sub-question on another, to supplement the general question. That way you don't stretch the boundaries of one site and still get the experts.