Are dwarfs now able to smooth and/or engrave both sides of a single-wide wall in the new version?
No, tiles are still limited to a single engraving. However! New to this version is the ability engrave constructed blocks, both wall and floor.
In that case, it would be better to have rooms and hallways be separated by two-wide walls. Also, what happens if you engrave a constructed wall and then deconstruct it? What happens if you then use the same blocks to construct a new wall?
It's a good question, so I tested it. 4 walls, made of a limestone block, a tower cap block, a silty clay boulder, and a highwood log. All 4 were engraved, then deconstructed. The building materials didn't change, so they did not remain engraved. I used them to again make walls, which were not engraved. I then engraved each of the walls with something new than what was there the first time.
So you cannot, for instance, build a large floor, engrave all of it, then move the engraved floor pieces to a new location. You can, however, build a large floor, engrave it all, and deconstruct the ones you don't like, starting the process over again until everything is as you like it. Or, you can force one dwarf to engrave forever for practice, just deconstructing the work and building a new floor again for a clean slate. Even better, make it out of slate!