Which is more professional in a dubbing of a documentary

I know when movies are dubbed, the original voice track it totally muted. But i have seen in some dubbed documentaries, that the original voice track is still there, but the volume is much lowered when compared to the dubbed voice. Whereas, in some other documentaries, original voice track is totally muted.
Which is more professional out of these two? Actually, i need to dub a documentary in native language.

I can’t speak as a professional here, this is only my opinion. But I think for documentaries it’s far more common to hear the original speech at a low volume and hear the dub right on top of it. I, personally, prefer this method since you can hear the inflection in the native speaker’s voice that you would lose otherwise. Though I understand that it does make it more difficult to understand, so there’s a trade-off.

I guess your best bet is to make two rough edits of a scene and ask a few people to compare/contrast them with you.

Another method is to start off the original voice at normal level, then fade it down to a low level as the voice-over translation begins.

Good call, Steve

That’s called the Auto Duck effect. Steve wrote a nice post about it here, it should greatly automate that process: