Signal (signal.org) is a great alternative to WhatsApp. It offers great security features and a clean desktop client that can be more convenient than QR-based browser coupling in many cases.
Yet, one thing I ran into recently is that it is not possible to work with the same account on a second Android device, say, a tablet.
Certainly, I can install the app and link my phone number, but then I am notified that the account is transfered to the new device and deactivated on the first one – which is the main device in my case.
I found no way to bypass this behavior using the core Signal app, but the security oriented fork of the client side app Molly can link an existing account in quite the same way that the Signal Desktop does.
The easiest way to obtain Molly ist to download and install F-Droid. You may need to temporarily(!) allow to install apk-files downloaded from your mobile browser. Make sure to revoke the browser permission after the installation using the app settings/permissions. Similarly, F-Droid should be allowed to install and maintain apps.
The second step is to add the molly repository to F-Droid as described here.
Once F-Droid knows about the repository, the installation is straightforward. Search for Molly in F-Droid (Molly Enhanced and security-focused fork of Signal, https://molly.im/), download and install.
When opening Molly the first time, decide if you wish to encrypt local data (passphrase upon start), then press continue (not: Transfer or restore account).
Enter your phone number, but do not press Continue.
Instead, look for a small text link saying Link to existing device. Click that.
Choose a device name. A QR code will show that you can scan using the Signal App of your first device (if your second device does not feature a camera, long-press he link provided and paste it in your browser, it should open with Signal on the device).
Running Molly on the same device in parallel seems out of scope at this point as the Signal app enforces using its internal QR scanner and the Molly link alone is not sufficient to connect both apps. I was not able to bypass this mechanism using a “hard screenshot” (photo) by a second trusted device.
Overall, linking accounts on a second trusted device seems to work fine and is also a nice way to quickly transfer data, look at photos on a larger screen, or simply receiving messages on this device.
Note that, if your second device is a shared one, be aware that messages may be shared with each person that can access the device. When in doubt, make sure to unlink devices using your primary Signal installation.