Bee,

Use a background that contrasts with the cats (you may have to photograph them separately, but I think you'll be able to work something out). And use your flash. The shadow that creates will help outline the lighter cat and bring the darker cat into view.

I sold an old digital camera on eBay this week and dug out a couple of photos I took to post as examples. And have been enjoying the "old home week" aspect of revisiting those photos. For example:

Yeah, this pit bull will tear your leg off. . . as soon as she's finished hanging out with her best friend, the sprinkler.

That tug-toy won't be bothering you any more, I've got it under control. . ."

and the heartbreaker,
Will You Love Me and Keep Me???