For tapered edge boards or dry lining there is a corner tape which is stuck with joint adhesive the same as joint tape.
They've got steel reinforced edges which when stuck, finish level with main part of board. (filling is required as with standard jointing where the taper starts and edge of corner tape begins) Clear??? Just google corner tape for dry lining and you'll see what I mean!
By putting a corner bead or corner tape over the tapered edges of an external corner, you are automaticaly joining the edges. Once the bead/tape is on, just fill out the tapered edges with filler to the flush level of the sheet of plasterboard.