I would not recommend to use two eye trackers simultaneously, since the NIR illumination can interfere. You will probably get ok results with only one eye tracker and a one-eye calibration, then use both eyes in the actual measurements.
To calculate gaze vectors with the Gaze SDK, use the Eye Position and Gaze Position points for each eye. They are both in the same coordinate system (UCS). See page 6 in “Tobii Gaze SDK Developers Guide – General Concepts”