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I doubt the circuit will work very well due to the TTL loading of the xtal oscillator input on pin 19 of the 89C51. Connecting 1clk of the 74LS74 to pin 18 of the 89C51 would be better and using a 74HC74 as well would be even better.
EDIT: Smarter still would be to program a divided 50% duty cycle clock to come out on P1.0 and use another pin for the ADC data buss bit0. That way you wouldn't need the 74LS74s at all.
I'm not that familiar with the 89C51, but reading the datasheet suggests that wouldn't be the case. It would however tie up the timer/Counter 2 as a clock generator so you couldn't use it for other things. It is possible to use Timer 2 as a baud-rate generator and a clock generator simultaneously, but the baud-rate and the P1.0 frequency would have to be the same.
But my main point was not to connect anything else besides the crystal and it's cap to pin 19 of the 89C51.