Is the Synaptics advanced settings available when you check for properties of the touchpad in Device Manager? See this to check if the registry shows the entry and also gives you steps. It is about...

It is the nature of using an external point and click device with a built in. Driver/IRQ conflict causes the internal built-in Point and Click to not be allowed to be used.

Your touchpad most likely is told to shut off with external point and click device connected.