My club allows one layer on the inside and one layer on the outside. This is not the case for all clubs. The layer on the outside leaves nice round holes when balls penetrate, even if the balsa behind it ripped away. The layer on the inside reduces the splintering of balsa and helps it maintain its strength. See your local retailer for details, your mileage may vary.
My club, and I believe most others including MWCI/IRCWCC use one on the inside and one on the outside like Gascan.
