Взаимное изменение EditText

Тебе нужно запомнить последнее значение и в методе afterTextChanged сравнить

if(lastValue != String.valueOf(s.toString())) {
   //тут твой код
}

Решил проблему обернув свой код в условие

if(productPieceEditText.hasFocus()) 

и аналогичное для второго поля, вопрос можно считать закрытым

   productPieceEditText.setText("0");
        productPackEditText.setText("0");

        productPackEditText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }
            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
              if(productPackEditText.hasFocus()) {
                  if (productPackEditText.getText().toString().equals("")) {
                      productPieceEditText.setText("0");
                  } else {
                      productPieceEditText.setText(String.valueOf(Integer.parseInt(productPackEditText.getText().toString()) * nomKoef));
                  }
              }
            }
            @Override
            public void afterTextChanged(Editable s) {
            }
        });
        productPieceEditText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }
            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
              if(productPieceEditText.hasFocus()) {
                  if (productPieceEditText.getText().toString().equals("")) {
                      productPackEditText.setText("0");
                  } else {
                      productPackEditText.setText(String.valueOf(Integer.parseInt(productPieceEditText.getText().toString()) % nomKoef));
                  }
              }
            }
            @Override
            public void afterTextChanged(Editable s) {

            }
        });

Есть такое решение:

public class MyEditText extends EditText {
private boolean lockWatcher = false;
private MyEditText myEditTextToChange;
private int index;
public MyEditText(Context context) {
    super(context);
}
public MyEditText(Context context, AttributeSet attrs) {
    super(context, attrs);
}
public MyEditText(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}
@Override
protected void onTextChanged(CharSequence text, int start, int lengthBefore, int lengthAfter) {
    super.onTextChanged(text, start, lengthBefore, lengthAfter);
    int nomKoef = 10;
    int result = getText().toString().equals("") ? 0 : Integer.valueOf(getText().toString());
    if (index == 0) result /= nomKoef;
    else result *= nomKoef;
    if (myEditTextToChange != null && !lockWatcher) myEditTextToChange.setText(String.valueOf(result));
}
public final void init(int index, final MyEditText myEditTextToChange) {
    this.index = index;
    this.myEditTextToChange = myEditTextToChange;
}
@Override
public void setText(CharSequence text, BufferType type) {
    lockWatcher = true;
    super.setText(text, type);
    lockWatcher = false;
}}

Activity:

public class MyActivity extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final MyEditText[] editTexts = new MyEditText[]{(MyEditText) findViewById(R.id.myEditTextPieces),
            (MyEditText) findViewById(R.id.myEditTextPacks)};
    for (int i = 0; i < editTexts.length; i++) {
        editTexts[i].setText("0");
        editTexts[i].init(i, editTexts[i == 0 ? 1 : 0]);
    }
}}

Layout:

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
              android:orientation="vertical" 
              android:layout_width="fill_parent" 
              android:layout_height="fill_parent"> 
    <com.example.TextEdit.MyEditText 
        android:layout_width="match_parent" 
        android:layout_height="wrap_content" 
        android:id="@+id/myEditTextPieces"/> 
    <com.example.TextEdit.MyEditText 
        android:layout_width="match_parent" 
        android:layout_height="wrap_content" 
        android:id="@+id/myEditTextPacks"/> 
</LinearLayout>