Optical image encryption technique has become increasingly important. Recently, double-encryption is introduced to enhance security. However, most coherent double-cryptosystems suffer from artefact noise, while the incoherent double-cryptosystem is not stable enough. This paper proposes a method for image double-encryption using speckle decorrelation properties in spatial and spectral domains. The plaintext is disguised as several different fake plaintexts that are separately encrypted through rotated random phase masks under incoherent illumination with different wavelengths. Since the speckle decorrelation properties in spatial and spectral domains, the plaintexts contained in the detected speckle images are multiplexed in a ciphertext. The security is enhanced since correct decryption can be achieved only when all the keys with spatial and spectral domains information are obtained. Simulations, experimental demonstrations and attack resistance tests are implemented with a simple incoherent cryptosystem that is easy to achieve in practical applications.