I have a product page with "microdata" and "json+ld" codes. Both of the codes refers to the same @id URI object (http://www.example.org/product#this) so I would expect to "mix/merge" both properties, but instead structured data testing tool shows 2 "individual" products so....
1- Does Google support using two syntax in the same page?
2- Is this well implemented? Can I refer two codes to the same object using itemId for microdata and @id for json+ld?
3- Can this damage my page in terms of structure data indexing?  
thanks
You can check it out using this code in test tool:
<div itemscope itemtype="http://schema.org/Product" itemid="http://www.example.org/product#this">
    <a itemprop="url" href="http://www.example.org/product">
    <div itemprop="name"><strong>Product Name</strong></div></a>
    <div itemprop="description">Product Description</div>
    <div itemprop="brand" itemscope itemtype="http://schema.org/Organization"><span itemprop="name">Product Brand</span></div>
    <div itemprop="offers" itemscope itemtype="http://schema.org/Offer">        <span itemprop="price">100</span><link itemprop="itemCondition" href="http://schema.org/NewCondition" /> New</div>
</div>
<script type="application/ld+json">
{
  "@context": "http://schema.org/",
  "@id": "http://www.example.org/product#this",
  "name": "Product Name",
  "@type": "Product",
  "image": "http://www.example.com/anvil_executive.jpg",
  "mpn": "925872",
  "brand": {
    "@type": "Thing",
    "name": "ACME"
  },
  "offers": {
    "@type": "Offer",
    "priceCurrency": "USD",
    "price": "119.99",
    "itemCondition": "http://schema.org/UsedCondition",
    "availability": "http://schema.org/InStock"
  }
}
</script>