You should be able to pose two naked, unmodified figures together, save as a scene, then reload and add any morphs, clothes, props, etc. you want, then tweak the poses to accomodate changes in figure height, weight, and so on.  I do it with Posette, so it should work (in theory) with any figures.

As far as purchased sets go, I have only seen matching images for this.  For example, one will be in a red body suit and the other in white.  The two thumbnails in the Library are the exact same thumb, with one listed as Red ## and the other as White ## with both ## being the same for that pairing.

Here is the product used in the example:

http://www.renderosity.com/mod/bcs/pin-me/83233