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 should integrate Monero, the way signal has integrated their shitty Monero fork. Then I can finally buy molly on molly on Molly.