Visual effect of 2 sections of content curving seamlessly into each other at opposite ends of their border

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

const primaryColor = Color(0xFFFDFDFD);
const secondaryColor = Color(0xFF252525);

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: Column(
          children: [
            Expanded(
              child: Stack(
                children: [
                  Container(color: secondaryColor),
                  Container(
                      decoration: BoxDecoration(
                          color: primaryColor,
                          borderRadius: BorderRadius.only(
                              bottomLeft: Radius.circular(88)))),
                ],
              ),
            ),
            Container(
              height: 200,
              child: Stack(
                children: [
                  Container(color: primaryColor),
                  Container(
                      decoration: BoxDecoration(
                          color: secondaryColor,
                          borderRadius: BorderRadius.only(
                              topRight: Radius.circular(88)))),
                ],
              ),
            ),
          ],
        ));
  }
}