Alright, Habakkuk 3:6.

(I'm just gonna start by pasting the Hebrew and LXX text here -- it's for my own benefit; it's not to trip you up or anything.)

עמד וימדד ארץ ראה ויתר גוים ויתפצצו הררי־עד שחו גבעות עולם הליכות עולם לו

ἔστη καὶ ἐσαλεύθη ἡ γῆ ἐπέβλεψεν καὶ διετάκη ἔθνη; διεθρύβη τὰ ὄρη βίᾳ, ἐτάκησαν βουνοὶ αἰώνιοι αὐτοῦ; πορείας αἰωνίας αὐτοῦ

In any case:

Like when Habakkuk 3:6 says (in the NASB translation) that "[t]he ancient hills collapsed. His ways are everlasting", and "aionios" is the word used in both cases?

You are aware that modern translations attempt to render the Hebrew text, only really consulting the Septuagint in disputed cases, right? So by no means is NASB a translation of the Greek, as your sentence here seems to imply.

(For the record, NASB's translation as a whole reads "Yes, the perpetual mountains were shattered, The ancient hills collapsed. His ways are everlasting.")

In any case: when we look at the Greek text itself, there are certain bizarre elements. For one, Hebrew עַד, which in its most common meaning is normally understand as "perpetuity," seems to have been misread by the Greek translator as עֹז, "strength, power." The first Greek verb here is from διαθρύπτω "shatter" or "break"; and consequently, this bit as a whole can be understood/translated as something like "the mountains were shattered by force" (perhaps more poetically, "the mountains were shattered violently").

After this, the two uses of Hebrew עולם ('olam) are both rendered as aionios. Now, the first of these uses -- in גבעות עולם ("'olam hills") -- seems pretty clearly parallel with the previous "perpetual mountains." (The Septuagint's rendering of this can again be discounted, as it misread the Hebrew here.)

In fact, if we adopt the emendation of the Hebrew text of Gen 49:26 that NRSV, NET, NAB(RE), NIV, etc. do, then we seem to have the exact same parallel.

Now, as we already saw, NASB rendered these lines together as "Yes, the perpetual mountains were shattered, The ancient hills collapsed."

However, other translations render the two phrases here as almost exactly synonymous. NET actually prefers to take both to be primarily suggesting antiquity, and thus renders

The ancient mountains disintegrate; the primeval hills are flattened.

Other translations, however, go with"eternal" and "everlasting":

NRSV:

The eternal mountains were shattered . . . the everlasting hills sank low

NJB:

And the eternal mountains are dislodged, the everlasting hills sink down

As mentioned, we can't really look to LXX to see how it handled the parallel, because it misunderstood the temporal word in the first phrase altogether. (Again, instead of "The ancient/eternal mountains were shattered," it had "the mountains were shattered by force.")

We only have the second phrase. Of course, though, if the Greek translators had understood it in the sense of "ancient" (as, again, NET rendered עולם there in English), there's a very obvious adjective that they would have used to denote this: ἀρχαῖος (archaios).

That being said though, I also mentioned that Gen 49:26 has the exact same parallel that Hab 3:6 does. And this can be seen in LXX. Here הררי־עד* [amended from הורי עד] and גבעות עולם are rendered as ὄρη μόνιμοι (ore monimoi) and θῖνες ἀέναος (thines aenaos). Monimos here is "immovable" -- or, more likely, "permanent."

NET: He travels on the ancient roads.

NJB: his pathway from of old

NRSV: along his ancient pathways

αἱ ὁδοὶ

ἐκ τοῦ αἰῶνος

πρὸ τοῦ αἰῶνος (Prov 8:22)