Here are the instructions to add an already published story to a publication:
257: How To Add Stories To Publications on Medium
Have you been asked to contribute to a Medium Publication? Do you want to write for Medium Publications?
You need to do the same with the other piece. It’s best to add to the publication before posting so it shows up in chronological order, Many publications won’t take any published pieces — only drafts — so it is always best to add to publication first. Let me know if you have any problems.