x1 (individual, mass, or population (set?)) and x2 (individual, mass, or population (set?)) are capable of mutually interbreeding/producing offspring with one another.
This word is symmetric under exchange of x1 with x2 (conversion by "se"). x1 need not be able to or actually fulfill the same roles in reproduction as x2 or vice-versa; actual instances of reproduction need not occur/be realized. The standard for what constitutes ability to interbreed (is it just genetics? Do physiological/anatomical features restricting, say, gestation and successful birth matter? Must the populations actually know of one another?) is ambiguous and left unspecified.