That can cause all sorts of problems like we have had in Denmark recently. If neither will say who was driving they could get off. Awful situation.
RIP
Or, depending on the circumstances, both could be charged as a joint enterprise.
Saw that a few years ago in a non-cycling driving accident.
Self-preservation took over, and all of a sudden one of them coughed to being the driver.
Inquiries can bottom out who was driving, DNA on a steering wheel airbag is a good one, or DNA/traceable fibres on the under surface of the driver's seatbelt, or fingerprints on the steering wheel/controls, or....
I've no idea if any of the above is relevant in this case.