picture in picture from one source clip

Hi, i'm hoping this is simple even though i cant find an example of it. Its probably best if i explain with example...
I have a shot of a choir starting with just 2 people's faces. the camera zooms out to show whole choir. i want to have a picture in picture effect keeping a close up of the 2 people's faces.
did i explain that well enough?
(there will not be any repeating. the faces will be 'zoomed in' and in the same 'time' as the rest of the choir).

i should also point out that the sequence is not one continuous clip - its made of a few clips already. so can you tell if its best i use group, or nest, merge...

thanks.