static async Task<string> DoIt()
{
    Random r = new Random();
    Task<string> t1 = Task.Delay(r.Next(3000)).ContinueWith(t => "one");
    Task<string> t2 = Task.Delay(r.Next(3000)).ContinueWith(t => "two");

    return await MagicFunction(t1, t2);
}

static Task<string> MagicFunction(Task<string> t1, Task<string> t2)
{
    TaskCompletionSource<string> res = new TaskCompletionSource<string>();
    Task.Factory.ContinueWhenAny(new Task<string>[] { t1, t2 }, t => res.SetResult(t.Result));

    return res.Task;
}

static async Task<string[]> DoIt()
{
    Random r = new Random();
    Task<string> t1 = Task.Delay(r.Next(3000)).ContinueWith(t => "one");
    Task<string> t2 = Task.Delay(r.Next(3000)).ContinueWith(t => "two");

    return await Task.WhenAll(t1, t2); // вернет string[]
}

static async Task<string> DoIt()
{
    Random r = new Random();
    Task<string> t1 = Task.Delay(r.Next(3000)).ContinueWith(t => "one");
    Task<string> t2 = Task.Delay(r.Next(3000)).ContinueWith(t => "two");

    var task = await Task.WhenAny(t1, t2); // вернет Task<string>
    return await task; // вернет string
}