You are not adding any security. The difficulty on obtaining two passwords is not double of the work of getting one password. If someone manages to compromise one password, you can assume they can compromise two or three.
If you have a keylogger installed, it will get whatever decryption passwords you have, no matter how many. In this case, encrypting it two or ten times adds nothing.
If you are targeted by a government agency, you may have to hand off the keys, so having an extra layer does not improve anything.
And if you somehow loses one password, you lose the entire drive. By considering security as the called CIA principles (Confidentiality, Integrity, Availability), the more layers you add, the less security you have - because of the decreased availability potential.