FocusNode textSecondFocusNode = new FocusNode();

    TextFormField textFirst = new TextFormField(
      onFieldSubmitted: (String value) {
        FocusScope.of(context).requestFocus(textSecondFocusNode);
      },
    );

    TextFormField textSecond = new TextFormField(
      focusNode: textSecondFocusNode,
    );

    // render textFirst and textSecond where you want