public static TO ConvertValue<TI, TO>(TI value) => (TO)Convert.ChangeType(value, typeof(TO));

public static void Deconstruct<TI, TO>(this IEnumerable<TI> src, out TO p1, out TO p2) {
    var e = src.GetEnumerator();
    p1 = e.MoveNext() ? ConvertValue<TI,TO>(e.Current) : default(TO);
    p2 = e.MoveNext() ? ConvertValue<TI,TO>(e.Current) : default(TO);
}

(int p1, int p2) = new int[] {  1, 2, 3, 4 };

Ext.Deconstruct(new int[] {  1, 2, 3 }, out p1, out p2);