How to enable dual bluetooth audio output on Garmin Zumo

Hi everyone

I own a Garmin Zumo590

On the unit settings it's possible to link one bluetooth audio output for navigation instructions and two audio output for multimedia (music)
When I use the option of TourGuides (very useful for me) only one audio output (Navigation Instructions output) can listen this.

But,...
In several cases, I supose for a unit bad function, the second audio output can listen Navigation Instructions as the first (both at same time)
It's impossible for me to reproduce this situation. It happens when unit wants.

Someone know how to enable this option permanent?

Thanks in advance