I'm not fully understanding what you're asking so I'll take a guess.
You have your main layer, you make a selection of the couple in order to adjust the background separate from them. There are a couple of ways; Duplicate the layer and carefully make a selection of the couple. Apply a layer mask to hide the background. You now have two layers, one with the couple above one showing the background. You can now apply any kind of adjustment layer (levels, saturation, etc) to the background layer without affecting the couple.
Now, if you want to make an adjustment to the couple you can apply a clipping adjustment layer to them without affecting the background.
There are other ways but this is probably the easiest to explain.
Here is a quick screen grab of what I'm describing. Hopefully this answers your question.
please log in to view hosted photos in full size.