Make an alt version of the ref pic with only the second person (or two alts for each person, if necessary). Do two openpose controlnets, once for each person.

Update: You don't even need the two references to be from the same image. https://yodayo.com/posts/6b0c6011-7a7b-4dfd-a63c-1c45619869e6/

(I am unable to track down the artist for the Haikyuu fanart I used as a reference here- please comment if you know who made it and I'll update this post.)