240v double pole breaker for two 120v circuits
The last guy got the 2-pole breaker thing right, the amperage wrong, and the GFCI wrong.  Here's what you need. 

The breaker needs to be correct for the smallest wire in the circuit.  If it's all 12 AWG (no 14 AWG extensions) then the breaker must be 20 amps or smaller.  A 30A breaker is wrong and should be removed from service immediately. 

Since it's a multi-wire branch circuit (MWBC), the circuit needs common shut-off, and with circuit breakers, that means either a 240V/2-pole breaker, or single breakers with handle-ties.  A handle-tie is a bit of plastic that sits in between the handles so they jointly shut-off.  It's sold by proper electrical supply houses; Big Box won't stock it. 

Since it's a bathroom, at least part of the circuit needs to be GFCI protected.  Single-pole GFCI breakers with handle ties won't work.  The only options are: 


A 2-pole, 120/240V GFCI breaker (expensive)
GFCI receptacles, making sure if you connect any additional loads on the "LOAD" side of the GFCI receptacle, that they use a dedicated hot-neutral pair separate from the MWBC shared neutral.