ToYieldInstruction.cs

public void Test()
{
    Observable.ReturnUnit()
            .SelectMany(_ => Observable.FromCoroutine(x => NestedCoroutine(x)))
            .Subscribe(_ => Debug.Log("Very Good:)"));
}

IEnumerator NestedCoroutine(CancellationToken cancel)
{
    yield return Observable.Timer(System.TimeSpan.FromSeconds(1))
        .FirstOrDefault()
        .ToYieldInstruction(cancel);
}