With this composite key mapping:
<class name="MyClass">
<composite-id>
<key-property name="Key1" />
<key-property name="Key2" />
</composite-id>
<property name="..." />
</class>
...you can use .Get like this:
var x = Session.Get<MyClass>(new MyClass() { Key1 = 'Foo', Key2 = 'Bar'});