Thanks Menno/Raghunathan for your valuable advise.
I will use separate crystals for both. Actually I am planning using two controllers on a single board. 89C51 generate some critical timing and receive data from a mechanical device and then filter as well as decode the data and send to 89C2051 by serially. This second controller again connected to another mechanical device and sends this data (coming from 89c51) to attached device. To send data to mechanical device, 89C2051 generate some critical timing. First mechanical device send 16 bytes in 450 milli seconds and another mechanical device complete the operation on these 16 bytes in 1 second and both these process will execute on same time and this sequence repeats 10 times continuously in one operation. If you can advise some other better option, please advise me. I will really appreciate you.

Thanks & Regards
Sanjeev Tyagi