how to calibrate 2 thunderbolt displays to the same level? is it possible?

hi, i bought one thunderboltdisplay last year and all works fine. Now o bought a second one this year. i wanted to calibrate them to the same level, but that is still impossible. i tried to adjust the color, warmness, etc. on both the same way, but one i darker than the other. can someone explain how to calibrate them to the same level? do i need external hardware for that?