Achieving successful dual/multiple monitor calibration on your system is hardware specific and differs with each and every set up. Generally it comes down to your LUT's (Look Up Tables) and if you can address them separately.
Not all questions will be answered for every different system in this article so we do ask that you do your own research and testing. We have provided notes on what to look for in the article below. You might want to refresh your knowledge on "How Monitor Profiles Work" as well as this will help to clarify some of the points discussed.
In general, on a Mac, multi monitor calibration just works. Mac video cards must support separate LUTs for each video output to be allowed to work with the Mac system. Just drag your profiling app to each screen in turn, perform the calibration, and you should be done.
On PC's, the key issue is to be aware of how many LUTs (Look Up Tables) your video card has, and to check that you can address each LUT individually. Unfortunately this information can be hard to find. We suggest contacting the maker of your video card directly to ask this question as typically computer stores usually won't know the answer to this.In general, if you have a fairly current video card and Windows 7 or above, dual monitor calibration usually works out of the box. It's pretty unusual to come across hardware without dual LUTs these days.
On PC's with older hardware/OSes, the easiest way to achieve multiple monitor calibration is to attach each monitor to a separate video card. Each card will have a separate LUT (almost all video cards do) and therefore it will be easy to associate a separate profile with each monitor.
If you have a dual headed video card, this situation is a lot more complicated. First, the card must offer separately addressable LUTs for each output. This differs with every type of video card. Strangely, many newer video cards do not offer this, whereas many older dual headed cards do.
Some claim not to but actually do - you often have to use separate tools to actually use the LUTs. Others claim to have multiple LUTs but they are not seen as such by the operating system and can't be accessed. Some recent driver updates for popular cards have disabled the second LUTs and so older drivers are required to get this working (which can have other negative side effects).
Windows Vista was highly unreliable in this area, and we strongly suggest you migrate to (at least) Windows 7 if you are using dual monitors.
Under Windows XP you can use this handy applet from Microsoft to load separate profiles into video cards that offer separate LUTs. You must install it in your start-up folder and use the /L switch to use it as a LUT loader.
In Vista and Windows 7 you must use the inbuilt Colour Control Panel to manage profiles under Vista, and generally this will let you associate separate profiles for separate devices but on Vista won't actually load the LUTs properly when the machine is booted. You can use the tools below to help with this and manually 'poke' profiles into LUTs. Windows 7 has a built in LUT loader that works with dual monitors.
X-Rite offer handy and free tools that can be useful to determine what is actually going on with your multi monitor set up, and in principle would be able to tell you whether you can individually load LUT tables for each of your displays separately.
These tools should help you to test and/or achieve a working multiple calibrated monitor scenario. In principle, if you have two separate LUT's this means the process should work, but in practice it may still fail, even if you have two separate calibrations stored in two separate LUTs, and you can associate two separate colour profiles for your screens in the Colour control panel applet.
It often does work, but under Vista there are usually still problems with the operating system failing to return the correct profile for the second screen when you, for instance, drag an image from one screen to the other. Under Vista, the only reliable solution is to use two separate video cards. Under XP it can sometimes be made to work but in our experience it isn't reliable. Often it will work until you go and re-calibrate and then it will get all confused again, requiring manual intervention. In the end, we recommend two video cards as this is the only really viable long term solution to the problem under Windows.
Dual monitor calibration generally works well using either the built in LUT loader (it's automatic if the profile associated with a device has LUT information in it), or you can use the LUT loaders that come with the various calibration packages. This is usually the better option as they tend to have checking systems to make sure nothing else tries to manipulate the LUT.
If you're having difficulties getting dual/multiple monitor calibration working, it's worth asking yourself if the effort is really worth it. Very often it is easy to use the second screen as just a browsing and palettes monitor, so colour management is not really required on the second screen.
You can of course still manually adjust your second monitor as best you can to match your primary screen - we do recommend you keep the brightness on your secondary screen significantly lower than your primary screen in this scenario thought to make sure you eye will white balance using the white point of the primary display.