Encrypt a file once with 50 characters password or twice with 25 characters? What better Encrypt the file once with 50 characters password or encrypt it twice each time with 25 characters password.
Layering encryption doesn't effectively concatenate the keys (despite what intuition may suggest). The attacker can still attack the two passwords separately, such as by using a meet-in-the-middle attack. This means the effective key space (that is, the number of possibilities for the combined password that the attacker must try) is much lower for the double-layer scheme.

Because the keys can be attacked separately, two half-length passwords do not offer as many effective combinations as one full-length password. If each character in the password has $n$ possibilities (for example, a standard keyboard might offer 26 uppercase + 26 lowercase + 10 numeric + 30 symbols = 92 characters), the 50-character password has $n^{50}$ possibilities, whereas the two 25-character passwords only provide security equivalent to $2 × n^{25}$ possibilities. Using our example numbers, that's $90^{50}$ for the single long password vs $2 \times 90^{25}$ for the two shorter ones.

For perspective, the 50-character password has about the square of the number of possibilities for two 25-character passwords. It's a lot more.