Edited:
Merge cols:
=TRANSPOSE(split(join(",",D2:D,E2:E),","))
merges 2 cols, not very clean, but works. (Same as here Stacking multiple columns on to one?)
Full formula:
=SORT(UNIQUE(FILTER(TRANSPOSE(split(join(",",D2:D,E2:E),","));NOT(ISBLANK(TRANSPOSE(split(join(",",D2:D,E2:E),",")))))); RANK(COUNTIF(TRANSPOSE(split(join(",",D2:D,E2:E),",")); UNIQUE(FILTER(TRANSPOSE(split(join(",",D2:D,E2:E),","));NOT(ISBLANK(TRANSPOSE(split(join(",",D2:D,E2:E),","))))))); COUNTIF(TRANSPOSE(split(join(",",D2:D,E2:E),",")); UNIQUE(FILTER(TRANSPOSE(split(join(",",D2:D,E2:E),","));NOT(ISBLANK(TRANSPOSE(split(join(",",D2:D,E2:E),","))))))); TRUE); FALSE)
The transpose could be done after the sort.