How to handle student plagiarism appropriately? I am nearing the end of teaching a course for first year students with a fellow student. 

We assigned a paper and 2 students have submitted almost identical papers. The policy is that if a student is caught they get a 0 and possible legal action on a case-by-case basis governed by department heads. 

I know that one of these students is a 4.0 students and it seems odd that she would either share or copy her paper from another student. I'll refer to her as student A. Student B's paper is about a 90% copy and normally cases where this happened it's evident that the students are friends and socialize during classes. I have never seen Student A and B talk to one another and I find it hard to believe student A would plagiarize off of another student. 

I do not want to have to fail both students if only 1 copied off the other without the other's knowledge. How can I appropriately handle the situation without seeming like there is a favoritism towards student A? Student B has been on the lower end of the class grade wise but is still a passing student.
Usually, the way to handle this is to ask both students to come in at the same time. And then in separate rooms, ask them to articulate one of the key points of the essay. It's usually simple to determine who wrote the paper and understood the ideas and who merely copied the text.

However, the key issue doesn't seem to be just whether B copied from A, but whether:


A allowed B to copy the material willingly (perhaps for remuneration)
A allowed it to be copied by B accidentally or through neglect (e.g., A left a draft on a public computer); or, 
B copied it by malicious action  (B hacked into A's computer).


(There is a fourth possibility: that both B and A used source C and both are plagiarists.)

It would be unfair to punish just B and let A go without proving A's innocence or culpability.

That all being said, untangling this is non-trivial and at this point, I'd declare it above my pay grade and send it up the chain of command to the university committee responsible for student ethics and misconduct (via the instructor on record if you're a teaching assistant).