I'm not cryptography expert (well, i'm here after all!) but i have this dubt:
Suppose i have 2 different keys:
And a plain text:
if i use AES128 on my plain text with my first key i get this:
now, if i use on this cipher text my second key i got this:
looks pretty safe, but i have a doubt: does this have the same security of a AES256? Or a single 128bit key that can decode my final text can exists?