Either way works, but 99% of the time I prefer to put each class in it's own file. The only time I break that rule is when I have a second class that is needed by (and only by) the first class.
As for calling it, you just need to include both Class1.php and Class2.php, then initiate the object, i.e.
$class2 = new Class2;
It might be a good time to get autoload going (if you're not already using it) as you'll find the more classes you write the including will grow to the point of ridiculousness.
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