Hi,
The easiest route is to create the second dropdown list and then in the Object > Field tab set its presence to "invisible" or "hidden".
Then in the first dropdown list you could put script in the exit event to change the presence of the second dropdown. For example:
if (this.rawValue == "Show me another list")
{
secondDropdown.presence = "visible";
}
else
{
secondDropdown.presence = "hidden";
}
Hope that helps,
Niall