I'm fairly new to the SEO industry so I can't give you an authoritative answer, but I can share some of my experience which seems relevant to your situation.
We have multiple retail websites with varying degrees of overlap of products on sale. One of our oldest domains specialises in fireplaces, but a couple of years ago we set up a general home products site which has a fireplace section featuring the same products as the specialist fireplace site. We wrote unique descriptions for all the fireplaces on new site.
When the time came to re-build the specialist fireplace site we ended up using the same Magento back-end as for our more general site. For the last 18 months both sites have been showing the same content for fireplaces, but with some differences in the content hierarchy. Currently the general site has a slightly higher domain authority and more external links, while the fireplaces site has links from a slightly higher number of domains.
The fireplace site took a nosedive in rankings for a couple of weeks after the re-launch, then not only recovered but gradually achieved higher rankings than before. The general home product site has never ranked as successfully for fireplace keywords, even though it was the originator of the content.
The difference in domain age may have a little to do with the difference in rankings, but other factors are pretty equal. From our experience it seems that where two related sites are sharing content Google prefers to show content from the site most relevant to the search rather than the site which originated the content.