I use Molly as my “second Signal”. I have a dual-SIM phone (personal and work) which I used to run with a separate Work profile (under Shelter) in order to have two instances of Signal running at once. This was cumbersome and chewed data like mad (I had to have another instance of my VPN running to cater for the Work profile apps; see here).
With Molly, it is a fork of Signal, seems quite a bit more paranoid (eg it always locks the app when you navigate away from it), and can encrypt the on-phone database with a separate passphrase (from the phone’s PIN/biometric) of your choosing. There is a FOSS form also in development.
I use it for my “work number Signal”, as opposed to my “personal number Signal”. Thus, it allows two different Signal numbers to function on the one phone at the same time, without needing the “Work Profile” trick using Shelter. The user experience is entirely comparable. I got the idea to do this from Michael Bazzell’s latest Extreme Privacy e-book, Mobile Devices.