You can't since square() depends on the result of even().
So you will have to merge them into one method with one loop. Inside that loop, do the 'even' stuff first and then the 'square' part.
[Edit]
void ProcessEvenAndSquare()
{
for (int i = 10; i < 20; i++)
{
listViewEven.Items.Add(new ListViewItem(i.ToString()));
int square = i * i;
listViewSquare.Items.Add(new ListViewItem(square.ToString()));
}
}
[/Edit]