This can absolutely be done easily, as long as you have a basic, fundamental understanding of video tracks and compositing and how to use the Ultra Key effect. The quick gist is this: Place your "background video" on Video track 1, place your green screen shot on video track 2 over the clip on V1 and then apply the Ultra Key effect to the green screen clip and key out the green. You can easily delete the audio from the green screen clip by Opt/Alt+clicking the audio clip and deleting it (or using the Drag Video Only icon from the source monitor when you initially add it to the timeline).
Try this article for a bit more detail:
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You want to have two video sources on the screen at the same time, and see both?
Put the background video on V1.
Put the foreground video on V2, directly above the background video on V2.
If the video on V2 was shot to be a foreground image (on a green screen, for example) then apply a keyer and key out the unwanted material.
If the video on V2 was not shot specifically to be foreground then either:
A) click on the video on V2 to select it, go to the Effects Controls Panel and adjust the Motion controls to reduce its size to reveal the background video or
B) Use the Pen tool under Opacity to draw a shape around the portion of V2 you want to see, and once you complete the matte the background video will be revealed.