Conventional flat loop heat pipes only possess one active zone, not satisfying the requirements for the special occasions. In this paper, an evaporator with a shared compensation chamber is developed for cooling the compact electronic devices. Its main feature is the compensation chamber shared by two heating surfaces, the shared compensation chamber plays a role in supplying to two wicks when two heating surfaces are heated simultaneously. Under the conditions of the favorable elevation with a slope of 10° in the gravity field and the working fluid inventory with 75%, its startup performance and characteristics of operation with variable heat load have been studied. At two different heat sink temperatures of 0°C and −5°C, the loop could start up successfully at different heat load arrangements. With the evaporator wall temperature below (90±2)°C, the transferred maximum heat load was 240W. Especially, start-up process with two heating surfaces was improved, and the temperature difference between two heating surfaces was less than 1°C. Otherwise, given the constant total heat load of 160W, the system demonstrated very fast response to alternative heat load applied to two heating surfaces.