PSA: For those who want signal on secondary phone, use molly.
Recently discovered this. Molly supports link with existing device just like on signal desktop. It even has benefit of getting entire chat history unlike signal desktop. Just restore the signal backup file during setup and then click link with existing device. Then scan with you primary phone. Beauty of open source.
Molly: https://molly.im/
I'm using molly for several months now it is really nice but recently I dive myself in XMPP and it is superior to molly/signal just because XMPP servers are auditable amd you can actually see if the server is using encryption or not while signal servers are closed source unfortunately, it's their only flaw
The signal source code is open source, it is hard to prove that the servers are running the source code that's published, and we know they have admitted to having source code they don't publish for anti-spam purposes.
But you could take the signal server source code and stand up your own signal servers today.
Your client encrypts and decrypts everything, so it is actually not a privacy concern regarding message content when we don't know what the server does.