Can multi-encrypted file be decrypted with one password only? Let's say I have a single file I want to encrypt with AES and I encrypt it twice (first encrypt the unencrypted file, then encrypt the encrypted file again). The first time I use password 'ABC', the second time I use 'DEF'. So in theory the attacker would have to decrypt it first by providing 'ABC' as password which would result in still encrypted garbage data - so they would provide 'DEF' and then they succeed. I wonder, though, if they may decrypt my file with only one password - perhaps there exists a password that is neither 'ABC' nor 'DEF' that successfully decrypts the file and they accidentally succeed with this password when bruteforcing. 

In other words:

ABC -> DEF -> plain file

XYZ (exists or not?) -> plain file

Does the existence of 'XYZ' depends on the algorithm(s) used and will it always/never/sometimes exist?
Theoretically speaking, there is a possibility that a specific hashing algorithm can hash two separate passwords that both result to the same hash, however the odds of such an event making much of a difference in a brute force attempt is minimal. In addition, if there are two separate file containers (one inside the other), a brute force attempt would stop after decryption the outside container. My recommendation for encryption would be AES-256 encryption with SHA-512 hashing as those are currently accepted as industry leading algorithms. Should you care to, veracrypt will give you an option nest encryptions such as Serpent-Twofish-AES which will in turn make decryption take more time.