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/
No, there’s no security benefit to this.
One goal of the signal foundation, has been making end-to-end encryption accessible for normal people. Keeping things simple one phone number per account is one method they used to do this. It does diminish an anonymity, it’s using phone numbers as a form of global ID. Which isn’t great.
So people could argue, that well having multiple signal accounts on the same device aren’t against the philosophy, they’re not going to spend any engineering effort making it happen.