I would say no, although when a portal page is rendered, the doview method of all the portlets on the page are invoked, so indirectly, I guess it is being called.
By the way, how do you know that other portlet will be on the page? Users can add and remove what they want.
Not to be rude, but it sounds like you're thinking like a Servlet/JSP programmer. Break the mold, and make the leap to portlets.
Maybe if there's a scenario you're tryint to implement, you could describe it, and we could give you some ideas.
-Cameron McKenzie