If you have the 32-bit Excel/32-bit JMP combination, something like the following will work:
dt = open("$SAMPLE_DATA/Big Class.jmp");
dt2 = open("$SAMPLE_DATA/Tiretread.jmp");
dt << Save database("DSN=Excel Files;DBQ=C:/Bigc.xlsb;", "BigC");
dt2 << Save database("DSN=Excel Files;DBQ=C:/Bigc.xlsb;", "Tires");
Brian