multiple item trade in

That is a question you should ask Apple directly. We are all end users on these forum so I doubt anyone here would know off the top of their heads. I would also research to see if you can find the Terms for the trade in program. You can reach Apple by calling AppleCare, let them know what you are asking and see if they can direct you to a resource.

AppleCare contact Information

I've not seen anyplace in a order page where you can add more than one item as a trade in. However, you could trade it the first item before hand and get the credit to an Apple gift card or your Apple ID. Then order the new Mac and trade in the second item. There you can have the credit from the first trade in applied to the purchase price.

However, as rkaufmann87 said, check with Apple personnel to verify the details.