test.js

SchemaName.findOne({
  _id: TheObjectId
}).lean().exec(theExtenderFunction); /* the .lean() is to make it JSON and modifiable */

function theExtenderFunction (err, firstResponse) {
  if (err)
    throw err; /* please handle your errors */

  AnotherSchema.findOne({
    _id: firstResponse.theOtherObjectId
  }).exec(function (error, secondResponse) {
    if (error)
      throw error; /* remember, handle your errors! */

    /* if you are using lodash or underscore, this is advisable:
    _.extend(firstResponse, {
      theOtherObjectId: secondResponse
    });
    */

    /* without underscore or lodash */
    firstResponse['theOtherObjectId'] = secondResponse;

    /* That will substitute firstResponse.theOtherObjectId with secondResponse data */

    /* now you can send firstResponse */

  });
}