TwoSix
Everyone's literal second-favorite poster
I'm not a FR lore guy, but IIRC, "Abeir-Toril" was the name of the world generally prior to 4e. The 4e FRCS introduced the idea of Abeir and Toril as co-extant worlds, separated from each other during the world's origins.I think the issue is that, at one point in the FR timeline - as chronicled in some 4e books - Abeir and Toril occupied the same space.
I don't think any of the books ever clarified exactly how that separation "worked", from a cosmological perspective. The vibe was definitely more of a "Fringe" like parallel dimension than "Oh, it's just in the Ethereal or Astral somewhere".