Two registered nurses at Berkshire Medical Center who for years helped explain their union's staffing quest to the public returned to the fray Tuesday, this time in advance of a statewide referendum question on the same topic. Amber VanBramer and Mark Brodeur joined a North Adams city councilor at a noon rally on North Street in favor of Question 1. The measure on the Nov. 6 ballot would set minimum staffing levels for RNs in Massachusetts hospitals.