I don't think that is possible. A cell can only hold one particular value (it can be a formula or a hard-coded value).
You can format the value in the cell to display in any format you choose, but it is formatting the current value. I don't know of any way you can format it to show a totally different value.
I think you will need to use a different cell to do that (you can always hide the cell so it is not visible).
So, what exactly is the reason you want it to behave this way?
If we have a clear understanding of why you want to do it this way, we may be able to provide alternative solutions.
For example, if you need the underlying value to be different, as it is being used in calculations somewhere, you can probably just adjust the calculation instead (to multiply or divide by 2).