Do you mean multiprocessing in the traditional use "one uC running several tasks at the same time" ?. From your post it seems you mean "multiple uCs running each a task and communicating with each other". In the first case, the '51 is not a good multiprocessor, look at the XA. In the second case you have several options depending on your need. Maximum flexibility and speed - forget master slave - use dual or quad port RAM (cypress semiconductor has a nice selection). Multimaster - and slow - use I2C. Master slave use 485 or IIC or - if serial w/o drivers. One way I have used with success in a many processors setup is to have a processor as the communications master and nothing else. The '51 in 9 bit mode is very efficient in not disturbing the slaves without cause.

Have fun,

Erik