Hybrid Energy Storage Systems (HESS) are gaining popularity due to their ability to compensate for the deficiencies of the conventional single energy storage solution. Battery-Double Layer Capacitor (DLC) is one of such HESS that is being adopted for different applications such as vehicle propulsion, auxiliary power unit and renewable energy storage. Real-time estimation of the states and parameters of such HESS is crucial for safe, efficient and optimal operation. In this paper, an online state-parameter estimation scheme is presented based on the electrical and thermal dynamics of the battery and DLC. The estimation scheme consists of two separate state-parameter estimators for battery and DLC each of which exploits a cascaded observer-based structure. The observers are designed based on sliding mode methodology. Theoretical verification of the overall state-parameter estimation is provided using Lyapunov's argument. Effectiveness of the scheme is verified via simulation studies.