A strange Problem. On HDMi I have crackls in the output. Bluetooth is without any problems. At first I assumed, that it works better even if I disconnect the bluetooth device.
I lost a lot of time in the past for that "simultanous audio output", without to find a stable solution without issues. Till I discovered, I didn't want a real simoultaous audio (never used two outputs simultanous). Basically I wanted sound though HDMI for video files and I2S (hifiberry) for music (mostly internet radio). And for this the "Audio Profiles" addon is perfect for me. With a bit of work in the addon settings, it's able to change automatically the audio output based on media type.