The primary site sells goods within a particular industry, the other a particular category within that industry. Whilst the smaller, secondary site is exclusive to a particular area of a market, I'm not quite sure how significant this level of optimisation is to warrant keeping it separate. Although its primary keyword is within the domain name and the content is obviously solely about this particular area. It is not promoted or updated very often and only has approx. 500-1000 visitors per month.
What is the best thing to do?
1. Leave both sites a they are.
2. Straight redirect from one to the other (I would redirect to the appropriate category of the larger site).
3. Redirect individual product pages from one site to the other.
4. Something else.