Half of the 25mm fan output is useless on a single-extruder UM2, so while I had the hot-end apart to replace the temp. sensor, I made a small piece of sheet-metal to deflect the second-extruder airflow toward the left-side single extruder.
The two left-most and right-most slots of the vertical piece are not in front of the fan, so blocking them is irrelevant. I used the second slot from the right.
I just cut a small rectangular piece of sheet aluminum and sanded it down to slide-fit into the second slot at the right end of the vertical piece that holds the top and bottom plate together. It's about an inch by a half-inch. See images below.
It is wedged in place by the screws that hold the top/vertical/bottom pieces together, which intrude into the second slot from the right.
After inserting and tightening the top and bottom screws, I bent it toward the left, where it directs most of the output from the right half of the fan toward the bowden stop - it does a better job of cooling the teflon block and the cold filament inside.