OK, fine...everyone wants to be serious this morning.
My rational for the "two plug" method - with the plug on the outside, it's a piece of cake to put it in and out when the boat is on the trailer. Which is the only place one should be putting the plug in/out. Retrieve the boat, pull the plug, strap down the transom, adjust the engine tilt, and put the plug back in. No need to hop in/out of the boat, crouch down under the motor well (getting your knees soaked on the certainly wet cockpit), all the while making a bunch of groaning old man noises.
The plug on the inside deals with "old man memory". Or the theoretical possibility that the plug comes out while underway...something I consider an old wives tale, but I'm paranoid enough to keep the spare down there.
Oh - and the absolute best plug made (and don't let anyone tell you otherwise :xnaughty ):
The Stainless T Plug. You can crank it down tighter, and it has no loop that could possibly be snagged on something while in the water.
And...the final bit o' coolness - head down to your local swimming pool supply store and pick up a tube of Aladdin Magic Lube. Apply it to the rubber on the plug.
This stuff was recommended to me many years back, for use on anything rubber - typically o-rings. It makes them last virtually forever, and provides a better seal. It just takes a tiny bit...I'm still using the first tube I bought.