Cells that can be inputs or outputs

Is there a way in Excel to have cells that can behave as either inputs or outputs?

For example:

A1:100%
A2: 60%
A3: 40%

I want A2 and A3 to have a relationship where they both must equal 100%. So that if I change A2 to 70%, A3 will automatically change to 30%. But if I change A3 to 25%, A2 will automatically change to 75%.

As far as I know I have to select one to be the dependant but I'd really like to write it in such a way that the user of this sheet can choose any one variable to manipulate.

Thanks!

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