I believe in symmetry. And rely on only the angle of attack to adjust the pitch. A wing with positive lift likes to pop you out of the water and stalls. That makes it harder to ride.
I could use some clarification from Hawaiis,..
but i think he is saying that the back foil is typically used to help control the pitch angle of the front foil.
the front foil will llikely have best trim angle at 0-2 degrees.
it will also generate at bit of forward pitch moment that will tends to nose dive the board.
placing the aft foil a good distance behind the main foil will allow it to resist any pitching moment from the front.
a bit of negative lift in the back will rotate the front to its desired sweet spot.
any deviation from that should help return the front to its desired equilibrium.
symmetric or asymmetric will both do the job, but symmetric will have the same efficiency when controlling pitch in both directions.
as a pitch control mechanism, the aft foil does not benefit from extra surface area if everything is trimmed properly. but im sure its provides extra stability (and drag) when things get out of control.