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/
Every time I want to try molly the version is too far behind to restore backup of signal. Currently my signal is at 6.37.2 and latest Molly is at 6.35.3