what type is the chart which you want to use that expression in?
=> Generally, it is possible to define one expression, give it any specific name and, in the next expression (but only within the same chart/ object), to refer to the result of that expression just by naming it.
Also, you don't have to display all expressions that you have defined. In a table_diagram for instance you can just switch a radiobutton on the "presentation" tab.
Are those columns in the same chart/object? Because if they are, you can use an expression in a different expression by referencing the expression's label.
For example: Assuming I have 2 different expressions, represented as two columns, labeled as: Sum and Average, and I want to multiply the two columns in a different expression, as a different column. In that case my new column's expression would be something like: ([Sum] + [Average]).
In conclusion, if you want to use an expression in a different expression, in the same chart, you can reference them by their labels, using [<label name>]. Generally QlikView will suggest you the expression name if you want to use this mechanism