The expenses you paid for the failed adoption and the expenses you paid for the successful adoption must both be applied towards one $10,000 credit. According to IRS Form 968:
"The amount of your adoption credit or exclusion is limited to $10,000 for each effort to adopt an eligible child. For this purpose, an attempt that leads to the successful adoption of a child and any unsuccessful attempt to adopt a different child are treated as one effort." See
http://www.irs.gov/pub/irs-pdf/p968.pdf
On a related note, I am researching whether the adoption credit can be taken even in the absence of a successful adoption. My wife and I had an adoption fall through last week. Form 968 says:
"If the eligible child is a U.S. citizen or resident, you can take the adoption credit or exclusion even if the adoption never becomes final." The form then becomes confusing because it determines when you can take the credit based on when it becomes final, which it just claimed was unnecessary in order to take the credit. I would appreciate hearing from anyone who claimed the credit for a failed adoption without getting nailed by the IRS.