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/
The Signal protocol is built in a way where you don't have to trust the server. The servers could be run by the NSA, it wouldn't matter. Especially now that the Signal protocol uses post-quantum cryptography.