The disclosure relates to a dual printhead assembly and a 3D printing apparatus. The dual printhead assembly includes a base, a printhead module and a switching module. The printhead module includes a positioning swing arm with a bearing disposed at the middle portion thereof, a first printhead unit and a second printhead unit fixed nearby two opposite edges of the bearing, respectively, and a fixing piece passing through the bearing to assemble the printhead module on the base. While a rotation shaft of the switching module is rotated to drive a first cam or a second cam to press an end or another end of an elastic sheet, a third or fourth connecting element disposed on the elastic sheet is driven to press an end or another end of the positioning swing arm, so as to adjust the height difference between the first printhead unit and the second printhead unit.