We're preparing to tile our shower/tub surround. I'll be removing existing cultured marble wall panels and removing the existing lathboard/plaster walls to a height of 5 feet above the tub flange. The plaster walls are about 3/4 inch or slightly more in thickness.

I want to set the cement board at a height so that the last course of tile goes over the edge of the cement board/plaster joint above. Any drawbacks and pitfalls to sandwiching a sheet of 1/2 inch cement board on top of 1/4 inch cement board? I'll use the 1 5/8 inch screws for adequate penetration. Would you tack the 1/4 inch cement board to the studs with a screw in each corner and the lay the 1/2 inch on top and run the screws every 8 inches through the two sheets? One alternative would be 1/4 inch lath strips on the stud faces but at 64 cents a running foot, it would be cheaper to just use 1/4 inch cement board under the 1/2 inch cement board and I am sure it would be stiffer as well.

To achieve a height I need to match the cement board to the plaster in the vertical plane I may need to shim another 1/16 or 1/8 inch at the stud facings. I haven't seen wood lattice that thin so I may be forced to resort to cardboard drywall shims for that purpose.

All suggestions and observations are welcomed. Thanks in advance.