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When the first copy is requested and you have printed the shipping label and mailed it, then you can click on the "Mailed" button, At this point, the system will allow you to then post the second copy.
I have had two copies of a book posted at the same time, but they have different IBSNs. Even though the book is the same, they are different versions, usually printed by different publishers, in different years. What you cannot do is post a book under a different ISBN then it has on its cover or inside page, as you risk the requestor giving you a RWAP, especially if the requestor is looking for a specific version under a specific ISBN.