You need to set it on the pool settings, the settings in vCenter is irrelevant and will not be used.
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Also, you need to make sure your VM has enough Video RAM to support multiple displays. The easiest thing to do is do what Linjo just told you, but in the Pool Setttings just select the biggest resolution you can get. Set the Video RAM on the VM to 128, that's an overkill but oh well, I just want it to work for you. After that SHUTDOWN the VM then manually start it again. You should be good to go, unless you are doing Windows 7.
You are throwing me off with your use of the word "stretch" stretching normally is a bad thing, thats where you get your taskbar going across both displays. If you want an "extended" display then yes you can do this, following the directions above.