If frequency switching is specified, BBSYN2 is the set of frequencies for the second scan of the switching loop. Frequency setting using FREQREF and any pcal defaults are not used for the second frequency set. BBSYN2 must be specified for this frequency set.
Frequency switching is not allowed when writing VEX files. Also, it is a little known and probably nearly unused option so it is not well tested and may well go away some day.