Saw a post somewhere, don't remember if it was Reddit, discord, or here. Anyway, the post seemed to indicate that engraving only has value in the direction towards the spot in which the dwarf who did the engraving was standing. So while there is no penalty for overlapping, I guess for min/maxing, it would seem double walling would be preferred to get full effect. Or maybe I read/inferred that wrong and both rooms get full wealth value, but the engraving can only be appreciated from the same side as where it was engraved from??
Engraving with walls is slightly complex. in order for an engraved wall to add its engraving to the room value, the room must contain, both the wall itself, and also the location the engraver was standing when it was engraved, this gives wall engravings a sort of directionality leading to people double walling so engravings can face into a room from both sides.
That's correct. Multi selecting the walls by default is still the optimal choice either way, since no room gets the value if you don't have walls selected, there's no penalty for overlap, and it's impossible to tell after the fact which way the engraving is facing.
This does add a weird caveat that you might want to give one of the rooms part of the hallway in case the wall was engraved from there. If your hallway is 1 tile wide, you'll also want the wall across the way (and any wall adjacent to that, in case of diagonal engraving,) because the opposing room could've been engraved from outside. That way you know that room will get the value of any hallway-facing engravings instead of wasting it, and you've probably engraved the hallway floor anyway. Min/maxing!
Or you could just give up on engraving rooms and use the exploit:
So you don't care about using glitches and Non-contiguous rooms. heres the way you break room value with minimal effort and resources.
1. Make a line of wall with 1 tile gap either side (if your engravers are really good this might only need to be 2-3 long)
2. Make the lines of floor either side of this wall out of the most valuable material you can (aluminum or platinum are great, steel and gold also work), and have your best engraver engrave them
3. Place doors on top of all those engraved tiles, what sort doesn't really matter, as they will be adding 10-100 value to 2-3k value floor tiles
4. For every room add the doors and the wall(this is a convenient rectangle). the wall makes the doors shareable, and the doors make the engraved floor shareable.
5. For optimal results hide this to the south east of your area, otherwise the zone icons can get rather confused