I'm working on one now, in which the husband and wife are standing close together in their yard with trees framing them; it includes them from the waist up. I also have a husband/wife portrait on my gallery page, and there's a lovely nontraditional example on another forum member, Catherine Muhly,'s page at
http://www.crmuhly.com/pages1024/index.html
If you're going to be working from photos which you will take, you can learn a lot by the way they position themselves relative to each other for the photo shoot, and that can be portrayed in the work. I would suspect that they have a fairly clear idea, if even on an unconscious level, of what they want the painting to look like.
Good luck with the commission!