I connected the reset pins, the clock pins, and on both of them I connected serial in 1 and 2 .
the serial in inputs of the second one are connected with the last output (OH) of the first register.
Now I cannot figure out how to send serial data to it from the arduino.
Which is the clock pin, which is the serial out pin, how can I access them?
it was just a the and gate input of the registers.
Not sure that make any sense.
I need two 8 bit registers, or four 4bit
? a seven segment display has seven segment hence you need seven outputs. Unless of course you don't actually have a seven segment display but a decoder chip driving the seven segment display, which is something different from what you said.