Friend Laurence Olivier gave them the nickname “The Celestials,” and Cole Porter’s musical Kiss Me Kate was inspired by their offstage battles, but married stage duo Alfred Lunt and Lynn Fontanne are now mostly forgotten. Actor-playwright Mark E. Lang co-stars with his wife Alison Murphy in this entertaining new play exploring their lives and careers, from their beginnings in the 1920s through a jaunt to London during World War II, to opening night of “The Visit” on Broadway in 1958.