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Just fit the quartz crystal and appropiate capacitors on pins 4 and 5 and link pin 4 of the first micro to pin 5 of the second micro.
Pin 4 of the second micro should be left unconnected.
As long as the swing voltage on pin 5 meat the specification of min 0,7Vcc for VIH and max 0,2Vcc for VIL everything is OK.
You can try Silvio's option of connecting the second 2051 directly to the first one, but I would consider manufacturer's recommendation to connect external clock source to 80C51-derivatives through a buffer gate .. see picture below ..