In this study, two single wood treatments—copper impregnation (I) with thermo-hydro treatment (T) and vice versa—were combined to determine the effectiveness against wood decay fungi. Pine (Pinus sylvestris L.) sapwood and birch (Betula spp.) boards were treated accordingly, then cut into small-size specimens and exposed to brown rot (Coniophora puteana) and white rot (Trametes versicolor) fungi to determine mass loss (ML). The specimens were collected from the board ends and middle part to detect copper distribution and, consequently, decay resistance. Wood leaching according to EN 84 was performed to evaluate the double treatment efficiency in outdoor conditions (use class 3, EN 335). The durability of the board ends and middle part differed in single-impregnated wood. The higher amount of copper at the board ends tended to cause lower ML. Double-treated I/T boards showed a similar tendency. In double-treated T/I wood, a higher copper amount at the board ends provided a higher ML in this part. Obviously, the interaction between thermal and chemical treatment has affected the wood structure, copper distribution and fixation. In leached pine and birch wood, the double treatments T/I and I/T were more efficient against brown rot than single thermo-hydro treatment and impregnation procedures, ensuring the durability class (DC) 2 (CEN/TS 15083–1). Regarding white rot, there was no pronounced difference between both double treatments and single treatments as they ensured DC1.