[MauranKilom]

Very basic example: ROT13 is a form of encryption. Applying ROT13 twice gives you plaintext.

It's of course not that trivial with better encryption algorithms. But before stacking encryption algorithms, try to first answer what you are trying to achieve (that application of a single algorithm does not).

[Moru]

It's more like doing ROT13 and ROT12 to strengthen the encryption. To decrypt you just need to do ROT1, not ROT-12 followed by ROT-13.