D-bus doesn't work if a different cache folder is specified

[1milano1]

Plan

Premium, developer

Country

Milano

Operating System

Ubuntu Desktop 16.04

My Question or Issue

I may use 2 Spotify instances (--mu flag) to redirect audio on different cards / rooms. But an issue raised as soon as I opened the first instance. If you use --mu, d-bus doesn't recognize the service / bus name even with a single instance:

$ spotify --mu=user1 --username="user1" --password="passwd"

$ dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.Play

Error org.freedesktop.DBus.Error.ServiceUnknown: The name org.mpris.MediaPlayer2.spotify was not provided by any .service files

I checked with ListNames if the name existed and checked with d-feet if it exists someway, without luck. Spotify correctly launches and ps aux | grep spotify returns 3 PIDs. D-bus correctly works if I open Spotify without --mu flag. How can I control it?