We banked a few leftover 2010 points and borrowed all our 2012 points for our September trip this year, along with using our 2011 points. Not theoretical at all!
If your trip spans two Use Years (2013/2014) you could also use banked points from 2012, current points from 2013, borrowed points from 2014 and borrowed points from 2015.
Just remember that banking and borrowing are final transactions that cannot be reversed. So you want to be real sure about the trip before you do both.
If your trip spans two Use Years (2013/2014) you could also use banked points from 2012, current points from 2013, borrowed points from 2014 and borrowed points from 2015.
If your trip is from July 20, 2013 through August 10, 2013, you could use 2011 and 2012 points for the July portion along with 2013 and 2014 points for the August portion.
If your trip is July 20 2013- Aug 10 2013
you could use 2011, 2012 and 2013 points for the July portion (in your 2012 UY)
And you can use 2012, 2013 and borrow 2014 points for the Aug portion (in your 2013 UY)
remember that traveling late in your UY puts you in a bind if you need to cancel, though.
for an aug UY, the banking window closes on march 31...so if you need to cancel those july dates after march 31, you need to make some quick adjustments or risk losing the pts completely...