because in C# you can't access to other Forms' Controls, but we know that static Members are accessible throw other classes. so you should define a static variable in first form to store items in it like this:
Code:
public static List<string> myList = new List<string>();
and somewhere else you should store data of listbox into this variable:
Code:
for(int i = 0; i < ListBox1.Items.Count; i++)
myList.Add(ListBox1.Items[i].ToString());
and in second form you can access this variable
Code:
for(int i = 0; i < Form1.myList.Length; i++)
ListBox2.Items.Add(Form1.myList[i]);