If the sorting scores are equivalent of two documents in an union search, we then sort them on the basis of their search index. So if there's a tie between documents of 3 collections, cars, motorbike and garages and the search query is:

{
  "union": true,
  "searches": [
    {
      "collection": "garages",
      ...
    },
    {
      "collection": "cars",
      ...
    },
    {
      "collection": "motorbike",
      ...
    }
  ]
}

the

garages

document will appear first in the response.