Two layers of 1" 6 pound wool will be 50% more insulation than one layer of 1" of 8 pound wool.
Unless you are going to make a dedicated welding forge, that will be plenty.
If you really want more insulation, use three layers. Three layers of 1" wool (about eight feet of wool) plus the refractory in a 12" pipe will make a robust forge with a good size chamber that barely gets hot on the outside.
For a welding forge, 2" of Hi-Z wool is the best choice.
TIPS:
1) When using multiple layers, just use a long strip in a spiral layer. Start and end to make the joint bump at the floor. You will be building up that area with bubble alumina (or some other flux resistant refractory) to make it flat anyway.
2) I highly recommend using 1" insboard for the ends. One layer will work fine, and two layers will make superbly insulated ends that are simple to make and install. This will greatly simplify lining the forge. 1/8" to 1/4" steel circle end-pieces over the insboard can be tack welded or attached with "L"-angle brackets to keep the board securely in place. Install the insboard when finishing the satanite/ITC lining. Make the cutouts in the boards and the steel end plates match, and line up when installing.
3)Measure the inside diameter of the shell and order the steel circles from online metals or any of the other suppliers. They will plasma cut them and you will only need to do a little trimming at the most. They are really cheap. See next tip for how to fit the ends.
4) (The measurements in this step are for a 12" shell and a 5-6" chamber. Make the same type calculations in the appropriate dimensions for other shell sizes)
Once you have the lining in, take a 12" piece of cardboard and cut it in a circle to fit the pipe just like the ends will. Draw on the thickness of the lining. Draw a line across the lining circle to mark the floor level. I like the floor to be about 4" wide on a 5-6" chamber and about 3" wide on a 4" chamber. Draw the front port so it is even with the floor and the desired width and height 3"X3" is good for a normal forge, and 4" wide by 3" tall is for a forge where you do wider objects. Cut out the port in the cardboard and you have the front end template. Mark the steel circle and cut out the port. The rear port is in the same position, but rarely needs to be as large. 3X3" is more than enough, with 2" high by 3" wide normal. Lay the steel ends over the insboard and cut them to match.
5) Install the ends ( you can just stick the insboard in place temporarily and fire the satanite. Start with short low firings, with a cooling after. Increase the length and temperature of the burn in several steps until the forge is run to a full internal glow for 5 minutes. Let cool and apply the ITC-100. Fire it to cure and then permanently install the ends.
6) Once the satanite and the ITC-100 are fired and cured, remove the end boards, put a little ITC-100 or satanite on the ends as mortar and put in the ends permanently. Install the metal ends over them. Add the floor layer by pouring/spooning it in through the end ports. Use a long spatula (long paint stir is perfect) to smooth out the floor and curve it up onto the sides smoothly, making it level with the end piece ports. Once the floor is in place and dried, fire the floor.
7) Welding a 3-4" shelf even with the front port makes a place to keep things from falling out when nit completely in the forge.
Welding two pieces of 1/2" pipe down the forge shell sides in a position that is parallel with the port bottom ( floor) will allow a movable tong/bar support. Make a square "U" from 3/8" round stock or rebar that will slip in the 1/2" pipe guides. It can be slid in and out as needed to support long things like tongs or sword bullets.
NOTE:
The poured floor is an important part of the forge. It is a 1" thick thermal mass that will help keep the forge temperature even, and help the blade absorb heat more evenly. While a firebrick will work , a poured layer will as as a better thermal mass. A piece of kiln shelving will work too, but a poured floor is still the best.
