Considering that you are getting this information 3rd hand at the supply house counter, I would pay little head to it.
Active 1's advice about checking the local codes is a wise idea. Although there in no physical difference electricaly speaking in having 2 circuits in either (2) 12/2 cables, or a single 12/2/2 cable, there may be some other factor. For instance, localy I am required to have all conductors identified by the phase they are on, and not 'technically' the way the local code is written to phase tape them for identification, they need to have the color in the pigment of the insulation. So that means I would either have to get red/white conductor romex, or use 12/2/2. (Or 14/2/2 depending on the circuit.) FYI CH makes a 3-wire AFCI breaker....
One on the breaker and the other to the bar.
I have no idea what purpose that would serve or why it would need to be done.... Sounds BOGUS......
