Multiple chains in fasta file from PDB

[bio_informatics]

Hi members,

I donwloaded fasta file for 1F88 pdb id.
I found that it has 2 sequence ids in it. One for Chain A and other B. I aligned them and found they are 100% identical.

Question(s):
- Why 2 or multiple sequences?
- Which one should I be using?

Thank you for your help in advance.