subjects - Can anyone help me to understand if the following sentence requires 'who' or 'whom'?

I am writing a story and would like to know which one is correct:

a) "...and it was impossible to know who was sheltering whom."

b) "...and it was impossible to know who was sheltering who."

I tried to apply the 'replace who by he/him' method, but I didn't figure out yet.

I appreciate your help.