Securely sharing end-to-end encrypted messages between devices is inherently difficult, especially combined with device verification. Note the 'secret chats' of Telegram are not shared across devices.
As far as I know, Matrix is the only network that has solved this problem. It even has cross-signing and federation.
Your best bet for the time being would be using mollyim-insider-android and that's what I use on my spare phone (Signal on main phone):
So far everything works as expected. Good luck!
Thanks for the rec!
Ages later I finally got around to migrating and it works great! :)
I been wanting this basic feature for years. I even stopped using it for awhile because of this lacking feature.
I would suggesting going to these threads on the official signal forums and giving them a thumbs up and leaving a comment on why you would like this feature.
https://community.signalusers.org/t/android-tablet-support/5590