I recently printed a very (diagonally) large part, with two materials that required the entire build volume to be reached with both nozzles. The TAZ dual extruder firmware currently limits the maximum Y to the same as it does for the single extruder, which keeps the first extruder over the print surface, but not the second extruder. I was able to achieve my print by changing the Y_MAX_POS to 326. This brings the Y axis bearing holder pretty close to the end of the smooth rod, so make sure this position works for you if you decide to do this. Any increase over the default 275 will increase your usable build volume. This also adds symmetry in when the Y is at its max, as the second extruder will be over the print surface, but not the first.