I have the following formula in a cell:

=VLOOKUP($E113,Eureka!$F$159:$V$182,6,0)+VLOOKUP($ E113,Eureka!$F$106:$V$155,6,0)

If the first condition cannot be found then I would like Excel to look for
the second; if the second condition is not found then I would -0- entered in
the column.

However, if the first condition IS found but NOT the second then I would
like only the first value to be entered in the cell (this would also need to
work in the opposite direction...FIND the second but NOT the first).

Could someone please help me to set this up? This is a major spreadsheet
with numerous columns and lines.