Is “Entlang” a two-way preposition? Dative and Accusative?
Some websites include “Entlang” as a two-way preposition and some does not.


This is an interesting question and seems to be rather complicated. As far as I can tell, entlang is most commonly placed after the noun and takes the accusative. This is how it is used in lesson 23.6, the only occurrence of enlang in the whole course.
Sie liefen die Straße entlang.
They walked along the street.

I should think for our level, that is enough.

However it is not as simple as that, it depends on whether entlang is before or after the noun, in some circumstances entlang can take either the dative or the genitive. The simplest explanation I found was in Wiktionary:

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