i was wondering if it was safe to clone the sd card of the first raspberry and put into the second one
are there any keys or files saved that should be unique to the device? (Except RSA keys for ssh, passwords)?
They don't need to be unique, no. I.e., you can clone the card and change or not change them afterward as you like. It is normal to use the same public key on multiple devices to save the hassle of keeping one for each. As for the sshd server private key, this is not really a big deal unless it is open to the internet beyond your LAN and you are worried about people cracking into one device then the other. However, the best protection against that is to prevent it from happening -- if they did it to one, simply having a different key probably won't stop them from using whatever technique they used a second time (which likely has nothing to do with the server keys).
An exception to that would be a static IP if you have one hardcoded and intend to plug in both pis on the network at the same time. Note also the ethernet MAC address will be different, although this should not have any consequences.