I know that we can use either the 12c or BAII+, but are we allowed to bring and use both calculators on exam day?
Yes.
Why would you?
I like to bring one calculator for each hand, and then attempt to answer two questions simultaneously. I think my 6th try on Level 1 is when I will finally pass.
I’m used to using the 12c, but like using the BAII+ for Combo/Permutation and DPB, just saves time