You are correct in calculating the shear in the chimnet girt at the end of the joist as a double shear problem. The wood under each side of the recieving mortise carries half of the 700 lbs of shear at the end of the joist. For the joint to fail, both sections of wood have to rip out. There is also extra protection in the fact that the girt does not end immediately past the mortise. Shear failure is a problem at the end of members because the cut end of the timber allows the wood fibers to move and seperate. The wood in the rest of the beam around an interior mortise helps restrain the shearing movement.