procedure TForm1.ImageEnMView1AcquireBitmap(Sender: TObject; ABitmap: TIEBitmap; DpiX, DpiY: Integer; var Handled: boolean);
var
  isFrontSide: Boolean;
begin
  Inc( fImagesAcquired );
  
  // Odd images = Front, Even images = Back
  isFrontSide := (fImagesAcquired mod 2) = 1;

  ...
end;