The "double speed" interpretation of a head-on impact is incorrect. The physics don't work out that way unless one of the people has far more mass (like so much mass that they aren't using this board anyway). Two people colliding head on while moving at 6.2mph is not at all like, e.g., one stationary person getting hit by a car moving at 12.4mph. It's much more similar to getting hit by a car at 6.2mph, or jogging into a tree at 6.2mph, both of which could kill you, but neither of which likely will.
You shouldn't be getting downvoted - you're right. Doubling your speed against a brick wall quadruples the energy involved in the collision, but two equivalent objects colliding at the original speed only doubles it. If they have the same or similar mass, the energy is absorbed equally by both, and so if two equal-sized people run into each other at 10 kph, each one of them will feel an impact equivalent to running into a brick wall at 10 kph, not 20 kph.
