The latter.
Identical footprint. Redundancy and division of capacity.
Law enforcement mostly on A. Fire & EMS mostly on B. A note below each Department heading in the PCWIN RR DB page indicates which simulcast that Department is typically on. Of course, can sometimes also be heard on the other, but not necessarily 100%. i.e. if a PCSO deputy is scanning Rural Metro, Rural Metro will stay on B as always but will also be pulled onto A so that deputy - who is logged onto A - can hear it.