gistfile1.cs

 
 // Task.WaitAll(_sProcesses.Select(p => _ReadFromConsoleAsync(p)).ToArray());
 
 private static async Task _ReadFromConsoleAsync(Process p)
{
   // Starst two tasks, waiting for any to complete and restarting one that just completed for next line.
   Task<String> stdoutTask, stderrTask;
   using (var stdoutReader = p.StandardOutput)
   using (var stderrReader = p.StandardError)
   {
      stdoutTask = stdoutReader.ReadLineAsync();
      stderrTask = stderrReader.ReadLineAsync();

      while (!p.HasExited)
      {
         var completedTask = await Task.WhenAny<String>(stdoutTask, stderrTask);
         var line = await completedTask;

         if (line == null) break; // EOP

         if (completedTask == stdoutTask)
         {
            Console.WriteLine(line);
            stdoutTask = stdoutReader.ReadLineAsync(); // restart
         }
         else
         {
            Console.Error.WriteLine(line);
            stderrTask = stderrReader.ReadLineAsync();
         }
      }
   }
}