Hi Naeem,
From your note it is clear that your master 89c51 is communicating with PC through serial and it is alredy using the TX and RX. so you can do one thing you can convert the received serial data into a 8 bit or 16 bit parallel data , which is taken out through ports and send to the other controllers in parallel.

Do like this It is my Theoratical Idea, not practically tried.
!) You recive data which is from pc to your master Mc
2) After receiving the data in master you convert the data in to an 8 bit or 16 bit data in parallel and feed them to the output ports.(if the ports are available)
3)Then you use another 89c51 which is an extra one not from your other 7 and recive the parallel data and convert it into serial..
you can communicate now with other controllers.Also try to do viceversa for the receiving data.
you can also do it with a parallel to serial data cnvertion using a shift register instead of that I think it will be better to use another 89c51 for that purpose.
with regards ,
mahesh krishnan