In some positions you need both B1 and B2. Me for example maintaining a twin engine full glasscockpit helicopter by myself at a linestation need both to be able to sign off loggbook remarks and performe sceduled maintenance. That doesn`t mean I know the avionics by heart but I`m certified to work accordin to the manuals. I dont do sheet metal work either but I make sure structural work is performed IAW MM and SRM and do the CRS when the sheetmetal guys are finished.