Merging solr indexes with duplicate keys - merging duplicate documents

Hi folks

We have a use case where i have 2 solr indexes with the same schema but
different field populated, for example:

Common schema:
<field name="url" type="text" />      // Unique key
<field name="product_name" type="text" />
<field name="image" type="text" />
<field name="brand" type="text" />
<field name="description" type="text" />

<field name="out_of_stock" type="boolean" />
<field name="num_likes" type="int" />
<field name="num_add_2_cart" type="int" />

Now i have one index which stores the information about products (first 5
fields). This index is built every 2 days.
I have a 2nd index which stores social signals (url + out_of_stock  +
num_likes + num_add_2_cart). This index is built every 2 hours and is used
for a near realtime boosting products.
The processes for building these indexes are independent, and for
operational management and for sake of reuse i would like to build these
indexes separately.

My question is, is there a convenient way of merging these 2 indexes (other
than applying document updates in a loop)? The IndexMergeTool from lucene
is not capable of applying document updates and would end up keeping either
first 5 field or last 3.

Thanks
Gagan