It all depends on the theme and topic of the sites. If they are related you can merge your sites into one site. This in effect reduces the amount of time you need to spend hunting down directory listings etc.
However if your sites are on different subjects then merging them only serves to dilute your content.
You state the 2 sites are unrelated subjects. Combining into one big site can only dilute the main theme (what is the main theme anyway for unrelated sites?).
Nonetheless (and importantly) a cross link would be good (if you were looking for garden hose, click here) because the keyword "hose" is a common factor (and likely the only common keyword)
What you seek in search engines is to get to the top for your search keyword. In your case that should bring to the top your most relevant webpage. Don't forget, not only do you want lots of visitors, but also relevant ones (panty buyers as opposed to garden lovers) to the webpage in question.