ADuC831 is upgrade to ADuC812. It has 62k flash program memory (ADuC812 has only 8k) and 2k on-chip SRAM which can be used as XRAM or as indirect addressable ram above standart 256 data RAM. When it's used as XRAM it's mapped at lowest 2k of the XRAM area. When you access it with MOVX command nothig happens with ports P0 and P2. You can read and write to them as usual. The question is what happens with them if you address memory above these 2k.
I've received reply from Analog Devices and they say that in this case everything works as a standart bus. You have address and data placed on P0 and P2. That means you can use on-chip RAM as XRAM and at the same time you may have XRAM mapped devices above 2k.
Moreover ADuC831 has more EEPROM and dual DPTR - very useful feature if you have to transfer blocks of data.