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);
    }
}

// the test

[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);
}

now if one fails, the tests name will tell you which one with out having to write fail messages.