these are hard to explain with out examples..

i want run a lookup from two different columns. if there is a value in
column A i want it to lookup the value for column B. but if the is no value
in column A i want to lookup the value for Column A using the value in column
B. i want to do this without the formula just looping round.

Column A and B are linked in the the lookup spreadsheet.

Hope this enough for someone to help.