How output JSON for multiple "Rest export" views?
One option would be to create a parent view and use Views Field View to reference the other 2 views displays.

For example, /car-stuff would contain 2 fields, "Cars" and "Tires", each one is a views field view field referencing the relevant json view display. For the parent view, you'd need to add REST Export Nested so the JSON of the child views would be proper JSON instead of a string.

Alternative Solutions

There may be a cleaner way to achieve what you're after.The easiest would be to make 2 calls to retrieve the JSON feeds and join them on the front end.

Depending on your requirements, you could also retrieve the specific tires for each car type.

{
    "cars": [
        { "nid": 1, "brand": "BMW", "weight": 100, "compatible_tire_nids": [100,101,102],
            "tires": [
              { "nid": 100, "brand": "Michelin" },
              { "nid": 101, "brand": "Good Year" },
              { "nid": 102, "brand": "Dunlop" }
            ]
        },
        { "nid": 2, "brand": "Mazda", "weight": 100, "compatible_tire_nids": [102],
            "tires": [
              { "nid": 102, "brand": "Dunlop" }           
            ]
        },
        { "nid": 3, "brand": "Volvo", "weight": 1000, "compatible_tire_nids": [103],
            "tires": [
              { "nid": 103, "brand": "Unknown" }
            ]
        }
    ]
}


The above would also use Views Field View, with the child view Tires having a contextual filter of nid. For an example of this approach, see the answer I provided here.