In this study, Dendrocalamus asper and Phyllostachys edulis (Moso) bamboo were subjected to a two-phase process of flattening and densification using a non-confined thermo-hydro mechanical (THM) system (composed of two hot plates without lateral limit) to redesign bamboo's microstructure and improve its performance for applications in panel manufacturing. Different softening procedures using pre-treatments with water under vacuum/pressure conditions using a vacuum of −650 mmHg and a pressure of approximately 3100 mmHg (V/P) and boiling treatment for 1 h at 100 °C (B) before flattening were studied. All groups were flattened, dried at room temperature, and densified at the same processing parameters. The microstructural, physical, and bending properties of the obtained flattened-densified bamboo using these two pretreatments were examined and compared with untreated-undensified (UN) bamboo and samples flattened-densified in an equilibrium moisture content (EQ). The results of bending tests, comparing to the un-processed bamboo, revealed that the flattening process combined with densification promote an average increase in the Modulus of Rupture (MOR), (56% and 66%), Modulus of Elasticity (MOE) (48% and 78%), Limit of Proportionality (41% and 86%), and Specific Energy (126% and −12%) in the case of D. asper, and Moso respectively. However, the applied process increased the thickness swelling (SW) and water absorption (WA). This negative effect was more pronounced in the case of Moso compared to D. asper. The flattened-densified Moso using any type of applied pretreatments presented an average of 64% of WA and 35% of SW, while these values for D. asper bamboo were 28% and 19% of WA and SW respectively. In addition, boiling the bamboo in water as a pre-treatment prior to densification reduces the cracks number compared to the other pre-treatments, resulting in higher MOR, MOE, and dimensional stability.