You need no second CPU. The 89C51Rd2 can program itself.
E.g. you place your custom bootloader at 0xE000..0xEFFF. This bootloader can do API calls to the original bootloader to do all the programming. Then programm the boot vector to 0xE0 and thus the custom bootloader was always called first after every reset. Then it can check, if reprogram must be done or if a valid program was resident at 0x0000. Then it jump to 0x0000 to execute your application.

On the Atmel web you can found example C code to do all the needed API calls.


Peter