bitcoin-cli attempts to read the credentials from $HOME/.bitcoin. Since user2 will have a different home directory, bitcoin-cli will not able to find the credentials.
You can either symlink .bitcoin from user1's home directory to user2's, or alias bitcoin-cli for user2 to use -datadir=user1home/.bitcoin.
You may need to play around with the read ...
In some cases, bitcoin-qt/bitcoind may regularly (and even gratuitously) flush the wallet , so it's possible that wallet.dat could be in an inconsistent state at the time you make your own copy.
The backupwallet RPC call should be a safe way to accomplish this.
I finally end up with this setting for my raspberry
content of the file bitcoind.service