Q:

MongoDB: How to "union all" results from same collection? 

query.1:
db.test.find({category_id:1}).sort({createAt:-1}).limit(5);

query.2:
db.test.find({category_id:2}).sort({createAt:-1}).limit(5);

What I want is use one query get results of query1+query2, and then sort the results by createAt. 

A:

You can use $facet aggregation here. 
db.test.aggregate([
  { "$facet": {
    "first": [
      { "$match": { "category_id": 1 }},
      { "$sort": { "createAt": -1 }},
      { "$limit": 5 }
    ],
    "second": [
      { "$match": { "category_id": 2 }},
      { "$sort": { "createAt": -1 }},
      { "$limit": 5 }
    ]
  }},
  { "$project": { "data": { "$concatArrays": ["$first", "$second"] }}},
  { "$unwind": "$data" },
  { "$replaceRoot": { "newRoot": "$data" }}
])

Update
Using simple javascript
const test1 = await db.test.find({ category_id: 1 }).sort({ createAt: -1 }).limit(5)

const test2 = await db.test.find({ category_id: 1 }).sort({ createAt: -1 }).limit(5)

const test = test1.concat(test2)

console.log(test)

