Do you ever consider that a customer might be more likely to click on a site dealing with one topic?
Yes I consider it. Because it's exactly what I do, given a choice. As a frequent Internet buyer.
Case in point, I had to buy a new data cable for the daughter's new cell phone last week. She'd broken the other one and of course you couldn't get that model anymore since it was 8 months old.
We use the usb data cords so that phonebook data can be backed up to a PC, have the easy ability to transfer photos down from phones, upload ringtones to our phones instead of paying $1.99 a pop, etc.
Now data cords are sold by almost everybody that has anything to do with cell phones. The place I chose carries a line of about half a dozen different add-ons for various cell models. Why? Because I could find what I was looking for quickly and because since they Specialize I have a sense that they'd be able to offer better support for these very specific items.
What makes link building easier on one site?
Because you get some latent link popularity from internal linking.
So a good link that is pointing to your main home page is going to get its weight spread out to other pages of the site by your natural internal linkage. The same goes for links pointing to an internal page. It doesn't stop right at the page being linked to.
The only way to accomplish this same thing with multiple sites is to have links from every page of every site linking to pages of your other site. In which case, they should have probably both been a single site to begin with.
Two sites that are basically unconnected means that you're going to have to gather just about twice as many links to have the same effect for this one single ranking factor.