Multiple Displays and Screen Sharing
This is probably not a problem with Screen Sharing, but a problem with iTeleport. Using screen sharing's default setup, I can see both of my iMac's displays from another computer:



And, it appears that iTeleport also supports multiple displays (see their FAQs).

I suggest that you check the connections to the extra display, and see if there are any settings for iTeleport.

You can also look at the Screen Sharing preferences by opening /System/Library/CoreServices/Screen Sharing.app and going to Screen Sharing-> Preferences. However, it doesn't appear that that allows you to change which display(s) are used.