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/
Molly now merged the long-awaited UnifiedPush feature that Signal refuses. This means the notifications go thru my server instead of Google’s.
That said, I hate the entire concept of only allowing one Android device but allo requiring an Android device with a SIM or you get no service. iOS is also supported but feeds into that duopoly, requirements to have a phone, & the freedom use whatever devices you want how you want.
I would prefer XMPP, but I have too many folks that refuse to move from Signal despite the conspiracies.