user37707: I have this Arduino Nano, Ethernet shield W5100 and RC522 as card reader. They are sharing pins D13, D12, D11, D10 is SS pin for W5100 D8 is SS pin for RC522. Problem is with pin D12, like it doesn't share it.
For example if I unplug D12 from W5100 RC522 reader works, if I wire it back it doesn't work anymore. W5100 works no matter if RC522 is wired or not, so problem would be somehow with pin D12.
Does anybody have some ideas, what I'm doing wrong right here?

user33608: I had almost exactly this situation with one of my projects.
The problem is that some Serial Peripheral Interface (SPI) devices don't properly put the MISO pin into high-impedance when SS is pulled high. I eventually resolved this by placing a tri-state buffer in the MISO line for each of the SPI devices.
I used a 74HC125 (4-channel tri-state buffer).
This is the page on Better SPI Bus Design in 3 steps that finally got me on the right track. I think the explanations are reasonably clear (at least compared with many other sites I found!).

37707: <p>I have this Arduino Nano, Ethernet shield W5100 and RC522 as card reader. They are sharing pins D13, D12, D11, D10 is SS pin for W5100 D8 is SS pin for RC522. Problem is with pin D12, like it doesn't share it.</p>

<p>For example if I unplug D12 from W5100 RC522 reader works, if I wire it back it doesn't work anymore. W5100 works no matter if RC522 is wired or not, so problem would be somehow with pin D12.</p>

<p>Does anybody have some ideas, what I'm doing wrong right here?</p>
 33608: <p>I had almost exactly this situation with one of my projects.</p>

<p>The problem is that some Serial Peripheral Interface (SPI) devices don't properly put the MISO pin into high-impedance when SS is pulled high. I eventually resolved this by placing a tri-state buffer in the MISO line for each of the SPI devices.</p>

<p>I used a <a href="https://assets.nexperia.com/documents/data-sheet/74HC_HCT125.pdf" rel="nofollow noreferrer">74HC125</a> (4-channel tri-state buffer).</p>

<p>This is the page on <a href="https://www.dorkbotpdx.org/blog/paul/better_spi_bus_design_in_3_steps" rel="nofollow noreferrer">Better SPI Bus Design in 3 steps</a> that finally got me on the right track. I think the explanations are reasonably clear (at least compared with many other sites I found!).</p>
