My 2 cents: Based on personal experience, if the couple appears to be mono, it is best to approach the woman or wife , Ms. CC, first. Make friends and feel the waters. In that way, you save face and avoid the unnecessary negative labels. Men are, by nature, more aggressive and Mr. CC might just jump in, without the poly setup you are desiring (and will end up cheating the lady partner or wify.)
IMO though im not one of the more experienced posters here.