How to launch automatically a daemon in a separate flatpak?

[Goffi]

Hello,

I'm creating flatpaks for Salut à Toi (https://salut-a-toi.org).

This project  works with a backend (a daemon) and many frontends, 
communicating with IPC (D-Bus is used most of time). I've successfuly 
created backend and frontends thanks to the help I've got on this list.

But for now I have to manually launch the backend, then the frontend, which 
is not really intuitive for a end-user. Is there a way to automatically 
launch the backend flatpak if it's not already as soon as a user request a 
frontend?

On a usual distribution, this can be accomplished with either a systemd 
.service file or a D-Bus service (like this: https://repos.goffi.org/sat/
raw-file/tip/misc/org.salutatoi.SAT.service).

Is there a way to do the same thing with flatpak?

Thanks
Goffi