Yes once is enough so long as enough salting volume is used and enough acid to properly titrate the xylene/aqueous solution. Second time you will only get a trace amount of alks, so it is really optional, SWIM usually skips it.
--------------------------------------------------*Kash's LSA Extraction* * Kash's Mescaline Extraction*------------------------------------------------------ All things I say are complete and utter ramblings of nonsense. Do not consider taking anything iterated from the depths of my subconsciousness rationally and/or seriously.