Increase to 1.4V per axis, any change?
Vref (Y) = 1.09V => 1.36A current limit / 2 motors = about 0.7A per motor
24V x 0.7V = 16W
Your Nema23 steppers can take up to 2,8A each, while the gshield can as a theoretical max wth active cooling provide 2,5A. But in terms of reliability 2A is max with active cooling, providing a little headroom.
Bottom line, the gshield is unable to take your steppers to the max and that is before we consider the two Y-motors sharing a single driver. The Controller for instance have 4A capacity per driver, and 4 drivers => the Y-steppers can have one driver each.