UberQueue.cs

class UberQueue<T> : IAsyncQueue<T>
{
    private IAsyncQueue<T>[] _queues;

    private Task<T>[] _current;

    public UberQueue(IAsyncQueue<T>[] queues)
    {
        _queues = queues;
        _current = new Task<T>[_queues.Length];
    }

    public async Task<T> DequeueAsync()
    {
        _current = _queues.Select((queue, index) => _current[index] ?? queue.DequeueAsync()).ToArray();

        var result = await Task.WhenAny(_current);

        for (var i = 0; i < _current.Length; i++)
        {
            if (_current[i] == result)
            {
                _current[i] = null;
            }
        }

        return result.Result;
    }
}

interface IAsyncQueue<T>
{
    Task<T> DequeueAsync();
}