To change the DPI you go to image > image size > then adjust the DPI at will. Upresing won't get you better quality, though.
As far as layering, you have to use the layer pallette. If you don't want to see the white background on the other one, you will need to remove it by hand using eraser, or remove it with a selection created by using the marquee tool or magic wand tool, or by select > color range and choosing white. If first piece is the "background" and Photoshop won't let you move files below it, just duplicate the background into a new layer... then you can change its layer order at will.