Complex sentence: Richard Widmark played Albert for six months until he was replaced by Alan Bunce, who co-starred with Lynch for the next twenty years not only in "Ethel and Albert", but in their radio counterparts in "The Couple Next Door".
Split into simpler sentences: Richard Widmark played Albert for six months until he was replaced by Alan Bunce. Bunce co-starred with Lynch for the next twenty years not only in "Ethel and Albert", but also as their radio counterparts in "The Couple Next Door".