The published installation instructions describe a single layer.
Use the single ThermalSheet layer shown in the published installation instructions. They describe 6 mm sheets with tightly butted edges, staggered seams and no overlap. They do not provide an approved stacked-layer assembly.
For the bonded tile/TempZone assembly, the sheet is bonded to the subfloor using the specified material. For the Environ floating-floor assembly, the instructions allow the sheet to float, with panels taped together and no overlap. Those are different installation methods, not permission to add a second layer.
If extra insulation or floor height is needed, choose an assembly documented for that purpose. Staggering seams and gluing two layers together does not, by itself, establish adequate support or approval for the finished floor.
Source: ThermalSheet installation instructions (assembly diagrams, Bonding Material and Install ThermalSheet).
