Personally, I would contact Etihad. Given that you are both Etihad passengers, travelling on the same flight, and that it is just the two of you, I would hope that it wouldn't be an issue. When booking the chauffeur, it doesn't ask how many people are travelling but as it is referenced to your PNR that information is probably provided to the chauffeur.
If you do call and get the all clear, I would ask for an email verification just as a backup or ask for a comment to be put on your booking. Alternatively, contact Etihad Guest via email - in my experience, they do usually respond fairly quickly.
Thanks Bazzi. I followed your advice and phoned them. They confirmed it would not be an issue. I keep my fingers crossed as we have booked 4 chauffeur drives, and it could be a bit of a problem if I could not get on the car. Their customer service does not sound very reliable. Maybe it is just because I spoke french to them ?