InterlockWith.cs

public static IEnumerable<T> InterlockWith<T>(this IEnumerable<T> seq1, IEnumerable<T> seq2)
    {
        Tuple<T[], int>[] metaSequences = new Tuple<T[], int>[2];
        metaSequences[0] = Tuple.Create(seq1.ToArray(), seq1.Count());
        metaSequences[1] = Tuple.Create(seq2.ToArray(), seq2.Count());
        var orderedMetas = metaSequences.OrderBy(x => x.Item2).ToArray();

        for (int i = 0; i < orderedMetas[0].Item2; i++)
        {
            yield return metaSequences[0].Item1[i];
            yield return metaSequences[1].Item1[i];
        }

        var remainingItems = orderedMetas[1].Item1.Skip(orderedMetas[0].Item2);
        foreach (var item in remainingItems)
        {
            yield return item;
        }
    }