SingleProducer.cs

public static class SingleProducer
{

    public static IObservable<T> Merge<T>(params IObservable<T>[] sequences)
    {
        IDisposable SubscribeObserver(IObserver<T> observer)
        {
            IDisposable SubscribeSequence(IObservable<T> sequence) => sequence.Subscribe(observer);

            foreach (var sequence in sequences)
                sequence.Subscribe(observer);

            return new CompositeDisposable(sequences.Select(SubscribeSequence));
        }

        return Observable.Create((Func<IObserver<T>, IDisposable>) SubscribeObserver);
    }

    public static IObservable<T> Merge<T>(IObservable<IObservable<T>> sequences) => sequences.SelectMany(sequence => sequence);
}