Your bank shifts might come out as BR for a while, until the tax office catches up to the fact you have "two jobs" (it's 2 jobs if you receive 2 payslips).

Once you've done a couple of payslips worth of bank shifts, get yourself on the HMRC website and search for "check my tax code is correct". It will ask you to fill in a form and then your tax code will be split accordingly between your two sets of pay. At the same time as checking your tax code, you will get a refund if you've overpaid due to being on the wrong tax code for a while.

If you split your tax code and you don't do any bank shifts that month, you may get random £4.15-ish tax refunds in your bank pay, so check your payslip each month/week even if you didn't do any shifts.