Dynamic cells

You will need to use VBA to achieve this if what you are asking is....
When you enter a value in A1, A2 adjusts so that A3 (the sum of A1 and A2) remains the same AND
when you enter a value in A2, A1 adjusts so that A3 remains the same.
Formula's reside in cells so when you manually enter a value, you automatically remove the formula.

You can either have user entered values or formulas in a cell but not both (unless you use VBA)

To use equations:

You could have user entries in A1 and A2 and calculations in B1 and B2

B1 = if(trim(len(A1)) = 0, 1- A2, "")
B2 = if(trim(len(A2)) = 0, 1- A1, "")

Edit: Too slow in typing...ChemistB is faster than this Chemical Engineer

with VBA:

Use worksheet_change event.

Press Alt + F11 to go to VBA editor, select your worksheet in the project browser on the left and paste following code

Private Sub Worksheet_Change(ByVal Target As Range)

On Error GoTo ErrorHandler
Application.EnableEvents = False
If Target.Address = "$A$1" Then Range("A2").Value = 1 - Target.Value
If Target.Address = "$A$2" Then Range("A1").Value = 1 - Target.Value

ErrorHandler:
Application.EnableEvents = True
    

End Sub

To use VBA
Right click on the worksheet tab and "View Code"
Copy and paste this code into the now open VBA editor

Private Sub worksheet_Change(ByVal Target As Range)

    If Target.Count = 1 And Target.Address = "$A$1" Then
        Application.EnableEvents = False
        Cells(2, 1).Value = Cells(3, 1).Value - Cells(1, 1).Value  'Cells(row,column) thus Cells(2,1) = A2
    End If
    
    If Target.Count = 1 And Target.Address = "$A$2" Then
        Application.EnableEvents = False
        Cells(1, 1).Value = Cells(3, 1).Value - Cells(2, 1).Value
    End If
    
    Application.EnableEvents = True
    
End Sub

Close the VBA editor (no need to save). Code is set for A1, A2 and A3 as per your example

Book1.xls

sorry I had trim and len swapped...that is why it didn't work


see attached with VBA and non-vba solution