Calculation based on entry into one of 2 cells
 

I am trying to use excel to calculate a result for cell one on entry
into cell two or conversely calculate the value of cell two on entry
into cell one. I have found a solution for this on a previous post I
located using targets and conditional calculations.

The difficulty I have though is I have entries into other cells that
are needed for the calculation and I cannot get the thing to
recalculate based on entries into the other cells or what the proper
logic should be. May on entry into these other cells I null out cell
one and two until all the cells and one of cell one or two have
entries.

What makes sense here??

Thanks for any help.