You need two master pages like you've attempted but you need to change a couple of settings. With master pages you have to tell the form which master page to use and when.
On your first master page, on the Master Page palette, set the Restrict Page Occurence to a min and max count of 1.
On the subform that flows to more pages look for the Pagination tab on the Object palette. Under "If Dataset Must Be Paginated" set OverFlow to your second master page.
To fix the page number you need to edit the script on the page count object. There is script on the # (and ## of the "page # of ##" control) - you have to select the number signs themselves to get at the script. Change the script on the second master page from this.rawValue = xfa.layout.page(this); to this.rawValue = xfa.layout.page(this) - 1;