Mary,
I think the two-adult composition is the hardest to manage. You have two equal masses that need to relate to each other and balance without becoming boring or dull. You also don't want to put too much attention on one party and neglect the other or cause speculation about dominance of either party (this is the problem with a lot of wedding photos). With a father and son portrait you should be safe with giving the father the dominance, but the son may steal the show anyway.
I love the composition in this linked painting. It's called "The Fallen Angels", painted in 1833 by Francois-Barthelemy Cibot. These men are nude, or arguably so, and I'm linking the painting instead of posting it.
http://www.joslyn.org/permcol/images/1995_60.jpg
I love the way the red sash and the snakes tie the two men together and give unity, flow and undulation. And can't you just hear them plotting what to do to get even?
I had a long conversation once with this painting, years ago, at the Jocelyn Art Museum in Omaha where I was the only one in the museum except for a sleeping guard. It had a lot to tell me about the nature of evil, beauty, ego... all the "good, bad and the ugly" nature of the universe.