It sounds like you want to offer identicle listings on both sites right? If so then what you should do is have the one site (The more successful one) have the listings static, and then on the other site you can use an iframe to show the listings, then the SE's wont spider that content but you can still write unique content for those pages.

The code if you arent familiar with inserting iframes is

<iframe src="wherever your listings page is"
width="whatever" height="whatever" frameborder="whatever"> I'm sorry but your browser doesn't support the IFRAME element.</iframe>

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