Actual both would have to show.
Yes, it is legal, an officer can hand out tickets to anyone based on another person's word, it does not have to be an off duty officer either, it can be an every day citizen.
However, both people must show in court the citing officer and the person making the accusation. The problem is many time when courts schedules hearings they are not aware there is another person involve so that person is not notified. So it will be up to the citing officer to remember the circumstance behind the ticket and try and arrange for that persons to show up at court. Instead of working around one officers schedule which he gets paid to show up in court, they will have to work around another officer's schedule which will not get paid to show up.
If I was you and the off duty officer showed up in court with his uniformation I would argue prejudusce against you since he was acting as citizen at the time not an officer of the court.
Also, which officers name is on the ticket the citing officer or the one who made the complaint.
Besides researching pacing, oall visual estimated since you could easily get the pacing evidence thrown out, the officer could always fall back to a visual estimate.