Because of the 8088, Commodore billed the BX series as 16-bit in the regular marketing balderdash they were swilling at the time. The "X", reportedly standing for "extra processor", was simply an upgraded version of the existing 600 and 700 computers; the 600 series computers with the extra 8088 received the BX moniker (BX128, BX256) and the 700 series computers, the CBMX designation.
In the BXes, the 6509 was relegated to I/O, keyboard scanning and display maintenance through an unknown method. Reports state that this allows users to use CP/M-86 and MS-DOS (!) on these units. The 700s and 600s themselves could accept an 8088 card to become BXes, and Steve Gray has found a NorthWest Music ad that confirms the cards do exist (apparently released as part of an inventory dump during the mid 1980's); this is confirmed by Bruce Faierson, NorthWest's former owner, who contributes the picture above. According to the ad, CP/M-86 ran quite well on these amphibious beasts.
There is also supposedly a Z80 card that runs at 4.5MHz, but its only mention is in the German service manual for these units (Ullrich); a Transactor clipping alleges that the Z8001, the processor used in the CBM 900, was also offered but no such card has ever been seen.