Goodwin, D. R. “On the Use of kai, in Hebrews x. 38.” Journal of the Society of Biblical Literature and Exegesis 5 (1885): 84-85.
In this brief article Goodwin argues that και in Hebrews 10:38 is not part of the author’s quotation from Hab 2:4 LXX. Instead the author quotes Hab 2:4 as two independent statements, separated by και and transposing the order. Thus the author of Hebrews is not furnishing a new subject, ο δικαιος, for υποστειληται; he is simply contrasting those who have faith with those who draw back.
I think the analysis is absolutely correct. In 2:12-13 the author also uses και to split a quotation, from Isaiah 8:17-18, thus making it into two different statements (although he does not transpose the order as he does in 10:38).