If they are related and the Bristol brand is a name shared between companies, they should stay together. Like Renault or Volvo, where cars and trucks are made by different companies, but using the same "brand".
But that seems a more complicated case indeed And Bristol being a city, it does not make clear if they really "share" a brand or both use the name of the city. Since they are not complete strangers, I think it is easier to keep both under the same make on the site.
Another edge case was this one: https://www.imcdb.org/vehicles_make-Fiberfab.html
The German company was founded by someone from the original US company, and since they shared some models under licence it is easier to have both listed as only make.