The fare system of MTR was finally merge on 28th September, 2008. People using "Single Journey Ticket" are not necessary to purchase another ticket when they travel between original MTR lines and ex-KCR lines.
Three "Interchange" station were merge and people are not necessary to pass through the "Gates" for "interchange" between two networks.
Tourists, normally use Single Journey Ticket or Tourist Day Pass are mostly benefit from this policy. You can directly choose the destination from the ticket vender machine, even you want to travel from MTR to ex-KCR.
Moreover, Tourist can purchase "Tourist Day Pass" at HK$55 for unlimited travel within most of the new MTR network for 24 consecutive hours. Moreover, an tourist version Octopus is also available for tourist, which contains 1 or 2 journey of Airport Express and a 3 consecutive days day pass for MTR network.
More information can be found at the official web site: http://www.mtr.com.hk/eng/train/products_index.html
Picture: MTR Tourist Day Pass (original from MTR Web site http://www.mtr.com.hk)