Learn English – the correct way to use ‘half as much …… as’ while comparing two things

The panda eats, shoots and leaves.

The syntax is correct: it relates an observation of a panda eating before shooting and leaving. However, the misplacement of the comma makes the sentence semantically incorrect, as the intention of the sentence should be that pandas eat shoots and leaves, not that this panda was shooting. (No offense to the Kung-Fu Panda, who may actually shoot.)

Comparisons of equality or similarity use as to introduce the entity you're comparing your topic to:

X is just as big as Y.
This answer is the same as Joe's ...

This is true even if the equality or similarity is denied; not takes the whole comparison into its scope:

... but it is not the same as Anne's.

Comparisons of inequality or difference use than:

X is bigger than Y.
This answer is less complete than Joe's ... but it is not less complete than Ann's.

Some comparisons use different prepositions: different itself uses to and from as well as than. But than and as don't flip.

The sentence you quote looks like a comparison of inequality; but the twice is a quantifier just like the negators (not) in my "answer" examples, and like those negators it takes the entire construction in its scope:

How many people die under 20 in Great Britain?
2x, where X equals the number of people who die under 20 in France.

It is thus a comparison of equality, and uses as ... as.