ConcatTwoOrderedList.cs

public IEnumerable<T> ConcatTwoOrderedArray<T>(IReadOnlyList<T> first, IReadOnlyList<T> second)
{
    dynamic dynamicFirst = first, dynamicSecond = second;
    for (int i = 0, j = 0; i < first.Count || j < second.Count;)
    {
        if (i < first.Count && (j >= second.Count || dynamicFirst[i] < dynamicSecond[j]))
            yield return first[i++];
        else
            yield return second[j++];
    }
}

public IEnumerable<T> ConcatTwoOrderedArray<T>(IEnumerable<T> first, IEnumerable<T> second)
{
    using(IEnumerator<T> firstWalker = first.GetEnumerator(), secondWalker = second.GetEnumerator())
    {
        var firstCanMoveNext = firstWalker.MoveNext();
        var secondCanMoveNext = secondWalker.MoveNext();
        while(firstCanMoveNext || secondCanMoveNext)
        {
            if (firstCanMoveNext && (!secondCanMoveNext || (dynamic)firstWalker.Current < secondWalker.Current))
            {
                yield return firstWalker.Current;
                firstCanMoveNext = firstWalker.MoveNext();
            }
            else
            {
                yield return secondWalker.Current;
                secondCanMoveNext = secondWalker.MoveNext();
            }
        }
    }
}