If you are the author of both questions (in this scenario you are not), then I think it's perfectly fair to just add an edit at the bottom stating something along the lines of:
This question was followed up with this question on Cross Validated.
When you're not the author, I would instead suggest posting it as a comment on the question. Hopefully get some upvotes for it, too, so that it gets guaranteed visibility. This is often used for intra-site linking as well.
In your case, since the question seems to be a followthrough from your answer. The Maths question is "Can this be done, what do I need to know?" kind of deal, and your answer addresses that. The Stats question is "How do I do this?" type of model. Since that's basically a lead-in from both the question and your answer to the Math question, I would suggest incorporating the link into your answer. That way, it not only links the two questions, but also adds more completion to your answer.