If that is the case, do as Johan said. If the images are on separate layers, just use the move tool to move it into position. If they are on the same layer, make a selection around the bottom one, then use the move tool to move it into place.
Is this what you want to do? You should separate circles. Select bottom part using Rectangular Marguee tool then Shift + Ctrl + J to cut selected portion and duplicate it on separate layer. Later follow instruction to make canvas wider and shorter.