This might not always happen but the first ferry and first express monorail might be held back to arrive at Magic Kingdom about 15 minutes before public opening. (Leaving Transportation & Ticket Center ca 25 minutes before public opening.) If you got to TTC 25 minutes before public opening, there might already be a second 700 passenger load and perhaps a third waiting to board the ferries and perhaps more than six monorail loads queued up on the upramps.
With two ferries the headway will be about 15 minutes. With 3 ferries docking becomes the limiting factor with a headway of about 12 minutes.