In the US, it varies wherever you go. Some places prefer having ground floor, floor one, floor two, some just start at floor one. If there is a B1 and B2, it would be called basement one and basement two or, even more clear, basement level one and basement level two.

Please note that in the UK, we only really have basements in large buildings like hospitals, big department stores and large tower blocks. What is termed as the first floor in the US, we'd refer to as the ground floor, then 1st floor, 2nd floor etc. Melanie

"1st basement" 2nd basement"