Simple linked list with two iterators
Use extension methods
I find you did well by implementing the EverySecondElementIterator as a separate class. However, I also think it would be better to turn it into an extension of IEnumerable&lt;T&gt; and use MoveNext instead of the internal Node.
Otherwise if someone suddenly comes up with the idea to skip every third element, you would need to modify the original SimpleList by creating another private class class and another property. We usually try to avoid modifying code that is tested and is working well. This would also violate the open/closed principle.
It's much safer and extendable to create a new and independent extension that you also can resue with any IEnumerable&lt;T&gt; and skip every-second-element of any collection.
Let the SimpleList be closed for modifications by not implementing anything as specific as that enumerator. It is already opend for extension by implementing the IEnumerable&lt;T&gt; interface. Make use of it.