Definitely not recommended, but if you have a roof that is gable without chimneys, skylight, dormers, walls, and flashing then you might be able to cheat otherwise in fact it would be chalking alone that would be keeping water out. On a roof chalking does not hold up to the continuous stress and harsh weather elements like shingles do.

Two layer systems are also very dangerous due to trapping water as well as trapped condensation resulting in mold concerns and hazards.