The whole basis of asynchronous communications is that is is asynchronous - the sender and receiver clocks don't need to be synchronised. That's why we have start and stop bits. If you want to do 8 bit with parity, you need to select the 9bit mode and calculate/check parity yourself. The 8051 has the P bit in the status register for the parity. Stop bits are effectively the idle time between successive characters. So if you pace the sending of the characters out of the 8051, you will be able to have as many stop bits as you require. Receiving two or more stop bits makes no difference.

You can always write a software uart to do whatever you want if necessary.