The fully passive activation of a flapping hydrofoil turbine is an emerging scheme for increasing the power efficiency of these devices and simplifying the system complexity owing to the use of a pitching controller. In this study, a dual-flapping hydrofoil turbine (DFHT) of which the front hydrofoil oscillates upstream and the rear hydrofoil oscillates downstream with respect to the corresponding axis with a new coupling mechanism is suggested to realize fully passive activation. The pitch and arm angular motions of the front and rear hydrofoils are simply linked by chain connections so that the turbine can start independently. A mechanical spring is adopted in order to enhance the recovery force to the middle position. It is found from an experimental evaluation in a circulating water channel that the coupling mechanism activates the DFHT fully passively with a nearly 90-degree phase difference at the middle positions where the power peak exists. Subsequently, zero-power zones are removed from the total power curve. An increase in the spring constant increases the flapping frequency and the self-starting velocity of the DFHT. Eventually, fully passive activation could contribute to improving the power efficiency, and the 90-degree phase difference could be utilized to reduce power fluctuations.