using Parrallel to Serial Shifting CD4021BE

I found a web site to use two of these wired parallel to each other. I know nothing about them. Im going to use them for 16 pushbuttons (inputs). So I take it there some sort of digital input to the arduino board. My question is if I use pushbutton wired to pin 1 of the CD4021BE that sends a signal to a digital arduino pin. If I have two of these in parrellel and I use the pushbutton wired to pin 1 onf the second CD4021BE how does the arduino board know the difference
One other question for input only does it matter if I use the above or can I use 2 74HC595

Why use an extra chip when all you need is a resistor ladder ?

HC595 is a 8-Bit Serial-In, Parallel-Out Shift Register
You may want (4/8/16) Bit Parallel-In, Serial-Out Shift Register

thanks for the replys. The resistor idea is ok I think for maybe 3 pushbuttons but any more than that if 2 are pushed at the same time it would get a little messy.
Im still unclear what one to use for my 16 inputs the HC595 or the CD402. Does it matter.
I can find info on the 8 bit CD4 but nothing on a 16 bit CD4

Assume you are refering to this tutorial:-

If you look at the bottom 4021 you will see its Serial Output Pin 3 feeds into the Serial In Pin11 of the Upper 4021.

When the Arduino Reads in the Serial data, its from the output Pin3 of the upper chip.
So the software reads in the 8 bits from the Upper 8 keys, and you decode them, as those 8 bits have been read Out of the upper 4021, the 8 bits from the lower 4021 replace them, so the Arduino reads in these 8 bits knowing they are from the lower 4021.

The 4021 seems common and cheap enough, any reason to use anything else ?

Thanks for that ricky101

Why use an extra chip when all you need is a resistor ladder?

Because it requires careful calibration and presumes that all the pushbuttons work perfectly - which they frequently do not.