How do I link two stories that are not a series or chapters in a book?

[AG31]

I have two stories. One is a prequel and the other a sequel. They are not the start of a series. They have different titles. I would like to put the URL for the companion at the bottom of each story, along with a brief explanation, but there's a timing problem. I can't put anything at the bottom of the first story until the second one is published.

Suggestions?
tia
ag