Chosen Cipher Text Security for Double Encryption
To understand the case of double encryption's CCA security, I read the paper Chosen-Ciphertext Security of Multiple
Encryption by Yevgeniy Dodis and Jonathan Katz in 2005.
With the help of this paper, now I'm able to understand why for the mentioned encryption scheme it will be no longer CCA secure when the key $k_2$ is known! Do give it a read for those having issues understanding CCA multiple encryption schemes.