Sub Demo()
Dim tw As Single, cw As Single, nw As Single
With Selection
  If .Columns.Count <> 2 Then Exit Sub
  tw = .Columns.Width
  cw = .Columns(1).Width
  nw = InputBox("The combined column width is: " & tw & vbCr & _
    "The first column width is: " & cw & vbCr & _
    "How wide should the first column be?", , cw)
  With .Columns(1)
    .ColumnWidth = .ColumnWidth * nw / cw
  End With
  With .Columns(2)
    .ColumnWidth = .ColumnWidth * (tw - nw) / (tw - cw)
  End With
End With
End Sub