I use my XPS 2720 as a second monitor through my HDMI input, and was wondering why my touch screen doesn't work?  Touch screen works fine as a primary monitor, but not when connected through the HDMI port.  I have an XPS 18 that is connected to a dell superspeed usb 3.0 video docking station, where I'm able to have a setup with three monitors.  I'd like both of my touch screens to work under this setup.  Please help.

Thanks,