map.js

use example;
db.dropDatabase();

db.coll1.insertMany([{
    created_at: new Date(),
    text: 'abc'
}, {
    created_at: new Date(),
    text: 'bbb'
}]);

db.coll2.insertMany([{
    created_at: new Date(),
    other: 'foo'
}]);

var pipeline = [
    {
        $addFields: {
            type: 'coll1'
        }
    },
    {
        "$unionWith": {
            coll: "coll2",
            pipeline: [
                {
                    $addFields: {
                        type: 'coll2'
                    }
                }
            ]
        },
    },
    {
        $group: {
            _id: "$type",
            type: { $first: '$type' },
            data: { $push: "$$ROOT" }
        }
    },
    {
        $unset: "_id"
    },
    {
        $unwind: "$data"
    },
    {
        $unset: "data.type"
    }
];

db.getCollection("coll1").aggregate(pipeline);