Hi,
the interview structure will be the same: PEI (15-20') + business case (30-40') + your questions (5'). The interviewers instead will differ: in the 1st round, you will face managers or principles, in the final round partners. The skills assessed will be consequently different: while in the 1st round your analytical and case structuring skills will be stressed, in the 2nd round partners will focus more on your fit with Bain and your common business sense.
Best,
Antonello