Ideally, you should be able to calibrate two monitors and have them look the same. However, many variables can affect the match between the monitors.
Hardware differences
Differences in make, model, and age of monitors will affect their similarity.
LED backlights perform differently than LCD's.
Laptop monitors may sacrifice color reproduction for lightweight and durable functionality.
If one monitor is older than the other, it may be slightly less bright.
Operating system support
Mac operating systems will support multiple profiles for multiple monitors
Windows XP offers the Color Control Applet to assign calibrations to the correct monitor
Windows Vista does not offer support
To get the best possible match between your monitors, calibrate them to the same color temperature, brightness and gamma settings if possible. Use the monitor with the lowest brightness as your common denominator for all the other monitors.