Nostalrius Begins - Quality wow vanilla realm (1.12)

Why do all hunters use the 2 Rend Blades?
is it 'cause the Enchant 15agi?

Everyone uses dual wields because the 25 agi 2H enchant doesn't come out until patch 1.10 (after AQ). And yeah, the 1 crit + 50 AP set bonus is prebis.

IMO it's not worth it to get both Rend swords. I tried to back in vanilla and it took months, a lot of ninjas from other players, a lot of hate from melee classes who needed the sword.

From Nost start up to a month or so ago Rend set swords were pretty common drop and most wars/rogues/hunters who played around that time had both, then someone complained its not Blizzlike and drop table was changed (for the worst). Now its easier to get ZG ones (hakkari mh + fang of faceless) which is also current bis (disregarding r14, which are same dps + sta bonus).

Back in the years, dual Bone Slicing Hatchet (http://db.vanillagaming.org/?item=18737) was the bis pre raid for hunts.

Bone Slicings are still very good, and could be said on par with dal'rend's, the availibility of the latter is though bit more complicated.
PvE Dual wield > Two hander.