On this page we read:
Deposit the money at the bank. A member of the bishopric deposits the donations. He should be accompanied by another Melchizedek Priesthood holder, usually a clerk. Where possible, use a bank with a 24-hour deposit box, and deposit the money on the same day it was collected, counted, and recorded. At the bank, both priesthood holders should witness the deposit being made.
To me it seems that that means the member of the bishopric and the MP holder should ride in the same car... but maybe I'm interpreting it too strictly and instead driving in separate cars is sufficient as long as the deposit is witnessed by both priesthood holders?