No, it generally isn't. I recently booked some flights on BA usinga AA miles, and while there were available seats, according to the BA site, AA could not book them. So, I asked them that very question and they told me that the inventory is not the same. In the end the AA agent put a request for BA to release a seat for my ticket, and it went through with no problem.
Originally Posted by SanDiego1K
Is there the same availability of awards whether using AA miles or BA miles? If so, you can check award availability on the BA website. It requires a BA mileage number and PIN. You log in, go to Executive Club and "redeeming miles". It takes you to a template where you put in the route you want to fly, the date, and the class. It shows you availability for a month at a time. I got two awards that I needed in December very easily.