要实现这一点，可以使用Row小部件，要扩展Text小部件，可以使用Expanded Package 它们。
完整的工作示例（基于您的代码）：

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ListView.separated(
          itemBuilder: (context, position) {
            return Container(
              height: 40.0,
              color: Colors.white,
              child: Text('Your text goes here'),
            );
          },
          separatorBuilder: (context, position) {
            return Row(
              crossAxisAlignment: CrossAxisAlignment.center,
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Expanded(
                  child: Container(
                    color: Colors.red,
                    child: Text(
                      'Divider 1',
                      textAlign: TextAlign.center,
                    ),
                  ),
                ),
                Expanded(
                  child: Container(
                    color: Colors.red,
                    child: Text(
                      'Divider 2',
                      textAlign: TextAlign.center,
                    ),
                  ),
                ),
              ],
            );
          },
          itemCount: 20,
        ),
      ),
    );
  }
}