I don't understand why one assert per test is superior

two-asserts-in-test.as

// the code

class eg
{
    public var width:int;
    public var height:int;

    public function eg(width:int, height:int)
    {
        this.width = width;
        this.height = height;
    }

    public function clone():eg
    {
        return new eg(width, height);
    }
}

// this test tests clone. If this test fails, clone fails, if it passes, clone passes

[Test]
public function a_clone_is_the_same_size_as_its_parent():void
{
    a = new eg(123, 321);
    b = a.clone();
    assertEquals(a.width, b.width);
    assertEquals(a.height, b.height);
}

// these tests test clone. If either test fails, clone fails, if both pass, clone passes

[Test]
public function a_clone_is_the_same_width_as_its_parent():void
{
    a = new eg(123, 321);
    b = a.clone();
    assertEquals(a.width, b.width);
}

[Test]
public function a_clone_is_the_same_height_as_its_parent():void
{
    a = new eg(123, 321);
    b = a.clone();
    assertEquals(a.height, b.height);
}