I had posted that I had two 99/500 offers back in November. You are doing it right.
A rep told me I could accept both offers. Feeling leary, I accepted the one I wanted (a 3rd party BofA-backed card) and held off on the other. After the money cleared and the inquiry dinged, I mailed the second offer back for the Plat. Plus, but was denied because I already had accepted that other card. To bad BofA reports so quickly. It cost me 4 inquiries, not to mention the two inquiries from the failed attempt at the fully secured card 3 mo. prior.
Letter states:
"Thank you for your request for the BofA Plat. Plus Visa [99/500 counter-offer].
Unfortunately, after careful review, we are unable to approve your request because the amount of credit extended to you by us and/or our affiliates is adequate......."