Sending a follow up email for a research assistant position
The answer written here assumes the position is still open and they are still searching for candidates.
Send a follow-up email to person A, cc'ing B.
Then, give them one week to reply (I mean, wait one week, not giving them a deadline :) ! ). If they do not react, try to call them on the phone (A or B, they seem to be equally involved in the decision process).
It is by far less time consuming for both parties (you and A, or you and B) to discuss such a thing via phone.