Complex sentence: Heartz suggests that at this time (1762) Joseph Haydn wrote his D major Horn Concerto for Leutgeb; the two may have been friends, as Haydn and his wife served as godparents for Leutgeb's child.
Split into simpler sentences: Heartz suggests that at this time (1762) Joseph Haydn wrote his Concerto in D, Hob. VIId/3D, Leutgeb; the two likely were friends, as Haydn and his wife served as godparents for Leutgeb's child.