Sometimes I have an image that I want to adjust to look like another in some way -- maybe the same brightness or color. This is easy to do because Aperture can display both at the same time and I can adjust one while using the other as a reference.

The first thing I do is press the S key to select Primary Only. This has no effect on adjustments (adjustments can be applied to only one image at time), but it does very conveniently make the currently selected image very obvious. Only the selected image has a white selection ring in this mode.

Here I have the upper image displayed as a reference and am adjusting the lower image:

This also works in full screen mode (F key) and with zoom on (Z key):

If I need to adjust two images at the same time, this is possible too. Clicking on an image changes the selection to that image and I make adjustments. This technique can be applied to as many images as I can cram on the screen.