Not sure if I'm right here but far as I'm reading from the manual this is how I understand it,
Originally Posted by 2012 Ford Focus Owner's Manual, page 19
unit of fuel
used for a given
That to me says the 'average fuel economy' will be calculated per trip setting. That is, gallons used divided by trip one or trip two, and not the total of trip one and two. So it'd be best to reset one trip and use that for average fuel economy and the other for whatever.