Node.js detecta cuando se terminan dos búsquedas de mangosta

Mongoose tiene soporte incorporado para promesas que brindan una forma limpia de esperar la finalización de múltiples operaciones de consulta asíncrona con Promise.all :

// Tell Mongoose to use the native Node.js promise library.
mongoose.Promise = global.Promise;

app.post('/init/autocomplete', function(req, res){
    var autocomplete = {
        companies: [],
        offices: []
    };

    // Call .exec() on each query without a callback to return its promise.
    Promise.all([Company.find({}).exec(), Office.find({}).exec()])
        .then(results => {
            // results is an array of the results of each promise, in order.
            autocomplete.companies = results[0].map(c => ({value: c.name}));
            autocomplete.offices = results[1].map(o => ({value: o.name}));
            res.json(autocomplete);
        })
        .catch(err => {
            throw err; // res.sendStatus(500) might be better here.
        });
});