It seems twinned vehicles are where they are the same chassis / car, but are sold under different badges.
Kind of like the Skoda Octavia / VW Passat in Europe - or the Suzuki Swift / Geo Metro / Pontiac Firefly / other versions of that car.
I notice Suzuki Swifts quite often because I see them so much on here - I also feel sorry for you in the US, because, when I went driving there, I hired a 'Saturn Ion', and it was me and someone else. As far as I was concerned, that was quite a big car, and they said 'are you sure that is enough for the both of you'? (!). In the UK, that would be a family size car, and would be fine for 4 people, comfortable for 3, and very comfortable for 2!. In the UK (and Europe), there are loads of small cars, such as the Nissan Micra, Peugeot 106, Renault Clio (=Nissan Micra so a 'twinned vehicle'), Ford Fiesta (although this has been getting bigger with every revision), and so on, whereas in the US (Georgia), the smallest car I saw was a new VW beetle (which is fairly large in my books).