My questions concerns the possible usage of "as well ... as" in the sense of "both ... and" or "not only ... but also". I know that "as well as" is a different idiom and means something along "and also".

The sentence of question is (the content and vocabulary itself is not important)
"They wanted to defend them as well against internal struggles as against the greed of France."
"They wanted to defend them both against internal struggles and against the greed of France."

This is intended as a translation of German "sowohl...als auch". My dictionary gives both possibilities (both...and / as well...as) but native speakers are opposed to the usage of "as well...as" and claim that only "as well as" exists.Of course, dictionaries contain mistakes and so I would like to receive comments of several native speakers.

Thanks in advance,
I don't think you can break as well as as you suggest.
I think this would be better:

"They wanted to defend them against internal struggles as well as against the greed of France."

You may want to review these quotations from literature, containing as well as against:

Active Service by Crane, Stephen - Chapter 2
college, as well as against the tremendous assaults of the Freshmen. Shamed by their own man, but knowing full well the right time and the wrong time for a ...

The Mysterious Island by Verne, Jules - Chapter 16
two legs as well as against savages with four. But, captain, will it not be best to explore every part of the island before undertaking anything else?" ...

The Monikins by Cooper, James Fenimore - Chapter 9
dialect, as well as against monikin taste, that would have been sufficient, under our opinions, even to overturn the government." "Tautology!" ...

The Great Boer War by Doyle, Arthur Conan - Chapter 3
its own tariff against the mother country, as well as against the rest of the world, and England be none the richer for the change. ...
www.literaturepost.com/chapter/4505.html - 31k - Cached - Similar pages

The Headsman by Cooper, James Fenimore - Chapter 2
find objections rising against thine own identity as well as against mine, friend Melchior. I am none other than Gaetano--the Gaetano of thy ...

The Pathfinder by Cooper, James Fenimore - Chapter 27
against me, as well as against another man; but then, I've never committed piracy, nor high treason, nor arson, nor any of them sort of things. ...