Cómo poner 2 imágenes en un solo lugar Flutter

Si su problema es acerca de la superposición de dos widgets, puede utilizar Stack para manejarlo.

Usted puede utilizar un Stack con Positioned para hacer esto: Aquí está el Ejemplo con los iconos:

class StackExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(),
      body:  new Container(
        padding: const EdgeInsets.all(8.0),
          height: 500.0,
          width: 500.0,
          // alignment: FractionalOffset.center,
          child: new Stack(
            //alignment:new Alignment(x, y)
            children: <Widget>[
              new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0)),
              new Positioned(
                left: 20.0,
                child: new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0)),
              ),
              new Positioned(
                left:40.0,
                child: new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0)),
              )

            ],
          ),
        ),
      )
    ;
  }
}


class StackExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(),
      body:  new Container(
        padding: const EdgeInsets.all(8.0),
          height: 500.0,
          width: 500.0,
          // alignment: FractionalOffset.center,
          child: new Stack(
            //alignment:new Alignment(x, y)
            children: <Widget>[
              new Container(
                decoration: new BoxDecoration(
                  borderRadius: BorderRadius.circular(25.0),
                  boxShadow: [
                    new BoxShadow(
                      blurRadius: 5.0,
                      offset: const Offset(3.0, 0.0),
                      color: Colors.grey,
                    )
                  ]
                ),
                child: new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0))),
              new Positioned(
                left: 20.0,
                child: new Container(
                  decoration: new BoxDecoration(
                  borderRadius: BorderRadius.circular(25.0),
                  boxShadow: [
                    new BoxShadow(
                      blurRadius: 5.0,
                      offset: const Offset(3.0, 0.0),
                      color: Colors.grey,
                    )
                  ]
                ),
                  child: new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0))),
              ),
              new Positioned(
                left:40.0,
                child: new Container(
                  decoration: new BoxDecoration(
                  borderRadius: BorderRadius.circular(25.0),
                  boxShadow: [
                    new BoxShadow(
                      blurRadius: 5.0,
                      offset: const Offset(3.0, 0.0),
                      color: Colors.grey,
                    )
                  ]
                )
                  ,child: new Icon(Icons.notifications, size: 36.0, color: const Color.fromRGBO(218, 165, 32, 1.0))),
              )

            ],
          ),
        ),
      )
    ;
  }
}