The Von Meck twins
The twins in the movie are played by Dennis and John Myers. Very attractive young men. I thought they were teens at the time but they were born in 1946. The movie was probably filmed early to mid 1970, so they were 23-24 years old at the time. Here is a link with some background information and a good shirtless photo.
http://www.britmovie.co.uk/forums/ask-film-question/107794-myers-twins-music-lovers.html
They were lean as rails! Hard to believe they are senior citizens now.