app.js

navigator.getDataStores('sergi2').then(function(stores) {
  console.log(stores.length)
  var store = stores[0];
  return store.add({ "blah": false }).then(function(id) {
    document.body.innerHTML = 'Inserted value in sergi2: ' + id;
  });
}, function(e) {
  document.body.innerHTML = 'Error inserting value in sergi2: ' + e;
  console.log(e);
});

navigator.getDataStores('sergi3').then(function(stores) {
  var store = stores[0];
  return store.add({ "blah": true }).then(function(id) {
    document.body.innerHTML = 'Inserted value in sergi2: ' + id;
  });
}, function(e) {
  document.body.innerHTML = 'Error inserting value in sergi2: ' + e;
  console.log(e);
});

manifest.webapp

{
  "version": "0.1.0",
    "name": "bl",

    "description": "A template with the bare minimum structure",
    "launch_path": "/index.html",
    "icons": {
      "16": "/img/icons/icon16x16.png",
      "48": "/img/icons/icon48x48.png",
      "60": "/img/icons/icon60x60.png",
      "128": "/img/icons/icon128x128.png"
    },
    "developer": {
      "name": "Your name",
      "url": "http://example.com"
    },
    "type": "certified",
    "permissions": {},
    "installs_allowed_from": [
      "*"
    ],
    "default_locale": "en",
    "datastores-owned": {
      "sergi1": {
        "access": "readwrite",
        "description": "my data store"
      },
      "sergi2": {
        "access": "readwrite",
        "description": "my data store2"
      },
      "sergi3": {
        "access": "readwrite",
        "description": "my data store2"
      }
    }
}