You can get off at either stop, but beware any checked baggage will be going to your expected destination and, of course, you must be booked thru to Kissimmee to ride south of Orlando. I assume you will know prior to boarding where you will be getting off (despite whatever your ticket may say), so you can check the baggage to the correct destination.
As for boarding, I would have an agent change my reservations to reflect my new origin point if it differs from your ticket. This is expecially important if you are in first-class, as the sleeper attendants may assume you are a no-show and the conductor may re-sell your accomodations to someone else. Not much chance of this in the few minutes from KIS to ORL, but why take the risk.