Coinjecting solvent with steam under steam-assisted gravity-drainage (SAGD) process to reduce the required steam amount for heavy-oil production has gained importance in recent years. The objective of this experimental study was to investigate the drainage mechanism of coinjecting light and heavy solvents to improve production performance. A 2D cross-sectional low-pressure scaled physical model was constructed. The model represented a half-symmetry cross section of a typical SAGD drainage in the Athabasca formation. Using an infrared camera, we visualized and recorded expansion of the steam chamber and temperature distribution. The fluid injection rate, pressure, and temperature, and produced-liquid volumes were also recorded. The results show that the relative condensation time of solvent and steam results in different production performances. Light solvent, delivered in the vapour phase to the entire fluid interface, reduces the bitumen viscosity along the whole vapour-chamber boundary, but it may build a thick gas blanket that may reduce the heat transfer from the high-temperature vapour chamber to the surrounding low-temperature bitumen. Coinjecting a suitable multicomponent-solvent mixture, including a heavy solvent, can enhance the production performance by altering the condensation dynamics of the light hydrocarbons. The conclusions from this study can be used to design suitable solvent mixtures and coinjection strategies to deliver a higher production rate, higher recovery factor with lower cumulative steam required/oil ratio (CSOR), and lower cumulative energy required for oil production (CEOR) from SAGD performance.