Stacking the floor 2 deep is going to slightly limit how wide your rooms can be, that last block or two where the ironwood beams turned red may only support 1 of the 2 in your stack. Also as you go up floors, you lose some width off the max you can do on the first floor, but you can switch to wood floors up higher if you don't mind that.

what I do can be summed up with
- hide columns of dirt in the walls to raise ground touch level to go higher
- use ironwood beams to support. The new angled ones are a giant help.
- keep an eye on how far across you can go at the lowest level. If you want bigger, you need to raise a central column of dirt, which again can be hidden with some care.
- you can also use trees as high reaching ground points, or a tree ON a column of dirt. If a fight or accident takes out the tree, things will fall.

Here is a tower i made, no mods, pre mistlands. The bottom level where it is wider has the raised ground ring inside the walls. You can see second ring up high is still stone floor, but note that this tower is very THIN. Its barely 1 good room per floor, about 7 or 8 2x2 tiles diameter.

https://steamcommunity.com/sharedfiles/filedetails/?id=2799499750

at low levels, the supports are simple:
https://steamcommunity.com/sharedfiles/filedetails/?id=2612860879
as you go up, you may need to start running the supports both directions making + signs or, if low on iron, trying some wonky diagonals in key spots or raising a vertical towards the middle. The higher you go, the more exotic/creative/ugly the support system can get. 3, maybe 4 floors is easy, and you can get 2 more underground easily.