using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

using System.Collections.Concurrent;

using System.Threading;

using System.Threading.Tasks;

public class Program

{

    public static readonly ClientDummy _client = new ClientDummy();

    public static async Task Main()

    {

        var results = new System.Collections.Concurrent.ConcurrentBag<string>();

        var args = Enumerable.Range(0, 10).Select(i => $"{i}").ToList();

        await Parallel.ForEachAsync( args, new ParallelOptions{ MaxDegreeOfParallelism = 2 }, (a, ct) => DoItAsync(a, results, ct)); 

        results.Dump();

    }

    public static async ValueTask DoItAsync( string someParam, ConcurrentBag<string> results, CancellationToken ct = default )

    {

        results.Add(await _client.Step2Async(await _client.Step1Async(someParam, ct), ct));

    }

}

​

public sealed class ClientDummy

{

    public async Task<string> Step1Async( string someParam, CancellationToken ct = default )

    {

        await Task.Delay(10, ct);

        return someParam + " from step 1";

    }

    public async Task<string> Step2Async( string someParam, CancellationToken ct = default )

    {

        await Task.Delay(10, ct);

        return $"Done: {someParam}";

    }

}