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/
100% Molly is amazing
Also you can run the normal signal client, and Molly on the same phone. To have two different signal accounts. If you use work profiles this could be four accounts etc