Per Datacolor support:
To calibrate and use two displays on one and the same Windows computer, you need a video card with two separate Lookup Tables (LUT).
But this exists only on newer and high quality video cards or if you have two separate graphic adapters in your system.
Several video cards offer two or more display connectors, but they only have one LUT. Therefore it's impossible to use two separate correction curves for these two connected screens.
They have proprietary Studio Match software which does the syncing, included in the Spyder 4 Elite software (top of the line).