full example

class ClassOne extends StatefulWidget {
  var person= 'myname';
  hey(){
    print('hey hello');
  }
  @override
  ClassOneState createState() =>  ClassOneState();
}

class ClassOneState extends State<ClassOne> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: ListView(
        children: <Widget>[

        ],
      ),
    );
  }
}

class ClassTwo extends StatefulWidget {
  @override
  _ClassTwoState createState() => new _ClassTwoState();
}

class _ClassTwoState extends State<ClassTwo> {

  var instone= ClassOne();
  _ClassTwoState(){
    instone.person="test";
  }

  @override
  void initState() {
    instone.person="test";
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: ListView(
        children: <Widget>[
        ],
      ),
    );
  }
}