Hi alexbor
Maybe someone else will correct me, but as far as I can tell you can't access Subsonic with Runeaudio.
Subsonic is a media server where you access the media from a web browser or other app that can handle web addresses.
Rune is not a web browser and can't access the server Subsonic provides. Rune is set up as a client/server that can control MPD using various media sources including network mounts. If you want to access remote media in the way you describe you need network attached storage.
It is, however, possible to implement a NAS on your second Raspberry Pi. Here is a random guide I just pulled off Google, but there are plenty of others:
https://eltechs.com/raspberry-pi-nas-guide/You may even be able to set up Subsonic on the same Pi as the NAS. Not sure though.
Whether or not a RPi is the best solution as a NAS I can't say.
Hope this helps.