When you calibrate multiple monitors to match each other, they may match closely but not exactly. You may especially notice a change in cases where you are using a mix of backlighting technologies.
In order to be sure that your monitors match each other as closely as possible, please be sure to set them to the same color temperature, gamma, and luminance settings. After you calibrate each monitor, determine which one has the lowest luminance and use that information as a reference for your other monitors. If you are using the Advanced mode in i1Match, you can use the "Open ICC profile" feature to import a profile from one monitor so that the software can automatically apply the settings to your additional monitors.
After profiling, you still may find that your monitors display very slight color variations due to hardware differences. Depending on your workflow, you may choose to designate one monitor as the color critical monitor and use the other for additional workspace.