Various methods for Enumerables.

EnumerableUtility.cs

using System.Collections;
using System.Collections.Generic;

public static class EnumerableUtility {
    public static TupleEnumerable<T1, T2> CombinedEnumerable<T1, T2>(this (IEnumerable<T1>, IEnumerable<T2>) me, int count1, int count2) {
        return new TupleEnumerable<T1, T2>(me.Item1, me.Item2, count1, count2);
    }

    public static TupleEnumerable<T1, T2> CombinedEnumerable<T1, T2>(this (IEnumerable<T1>, IEnumerable<T2>) me) {
        return new TupleEnumerable<T1, T2>(me.Item1, me.Item2);
    }

    public class TupleEnumerable<T1, T2> : IEnumerable<(T1, T2)> {
        public TupleEnumerable(IEnumerable<T1> enumerable1, IEnumerable<T2> enumerable2, int count1, int count2, bool lockstep = true) {
            this.enumerable1 = enumerable1;
            this.enumerable2 = enumerable2;

            if (lockstep)
                CheckCount(count1, count2);
        }

        public TupleEnumerable(IEnumerable<T1> enumerable1, IEnumerable<T2> enumerable2, bool lockstep = true) {
            this.enumerable1 = enumerable1;
            this.enumerable2 = enumerable2;

            if (lockstep)
                CheckCount(enumerable1.Count(), enumerable2.Count());
        }

        public IEnumerator<(T1, T2)> GetEnumerator() => new TupleEnumerator<T1, T2>(enumerable1, enumerable2);

        IEnumerable<T1> enumerable1;
        IEnumerable<T2> enumerable2;
        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

        void CheckCount(int count1, int count2) {
            if (count1 != count2) {
                throw new System.Exception("Enumberable 1 and Enumberable 2 are not of same length, but they should be.");
            }
        }
    }

    class TupleEnumerator<T1, T2> : IEnumerator<(T1, T2)> {
        public TupleEnumerator(IEnumerable<T1> enumerable1, IEnumerable<T2> enumerable2) {
            enumerator1 = enumerable1.GetEnumerator();
            enumerator2 = enumerable2.GetEnumerator();
        }

        public (T1, T2) Current => (enumerator1.Current, enumerator2.Current);

        public void Dispose() {
            enumerator1.Dispose();
            enumerator2.Dispose();
        }

        public bool MoveNext() {
            return enumerator1.MoveNext() && enumerator2.MoveNext();
        }

        public void Reset() {
            enumerator1.Reset();
            enumerator2.Reset();
        }

        IEnumerator<T1> enumerator1;
        IEnumerator<T2> enumerator2;

        object IEnumerator.Current => Current;
    }

    static int Count(this IEnumerable me) {
        var ret = 0;
        foreach (var thisOne in me) {
            ret++;
        }
        return ret;
    }
}