In order to run several Signal desktop instances in parallel, you can pass a –user-data-dir=<path> commandline option to signal-desktop.

First find out in which way your existing Signal instance was installed: For Flatpak, run

flatpak list | grep signal-desktop

For Snap, run

snap list signal-desktop

For distribution package run

dpkg -l signal-desktop

When Signal Desktop is installed via flatpak, do the following:

1. Copy /var/lib/flatpak/app/org.signal.Signal/current/active/files/share/applications/org.signal.Signal.desktop to ~/.local/share/applications/Signal2.desktop
2. Then edit ~/.local/share/applications/Signal2.desktop and change lines 2-3 as follows (replace <user> with your username):

   Name=Signal 2
   Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=signal-desktop --file-forwarding org.signal.Signal --user-data-dir=/home/<user>/.var/app/org.signal.Signal/config/Signal2 @@u %U @@

Afterwards, you should find the second instance of Signal Desktop as „Signal 2“ in your application launcher.

Sometimes Signal asks you to verify that you're a human by solving a captcha. The link after the captcha is always opened by the default Signal client profile. You can change this temporarily by setting the Exec value of the .desktop file of the default Signal client:

In /var/lib/flatpak/app/org.signal.Signal/x86_64/stable/active/files/share/applications/org.signal.Signal.desktop:

Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=signal-desktop --file-forwarding org.signal.Signal --user-data-dir=/home/<user>/.var/app/org.signal.Signal/config/Signal2 @@u %U @@

When Signal Desktop is installed via snap, do the following:

1. Copy /var/lib/snapd/desktop/applications/signal-desktop_signal-desktop.desktop to ~/.local/share/applications/Signal2.desktop
2. Then edit ~/.local/share/applications/Signal2.desktop and change lines 2-3 as follows (replace <user> with your username):

   Name=Signal 2
   Exec=env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/signal-desktop_signal-desktop.desktop /snap/bin/signal-desktop --no-sandbox --user-data-dir=/home/<user>/snap/signal-desktop/current/.config/Signal2 %U

Afterwards, you should find the second instance of Signal Desktop as „Signal 2“ in your application launcher.