How to test a second parameter in a PHPUnit mock object

This is what I have:

$observer = $this->getMock('SomeObserverClass', array('method'));
$observer->expects($this->once())
         ->method('method')
         ->with($this->equalTo($arg1));

But the method should take two parameters. I am only testing that the first parameter is being passed correctly (as $arg1).

How do test the second parameter?