I have a couple sites that I have "branded" in similar ways. They're both connected to me directly with a unique name......so I'm known at various forums by this name, comment on blogs with this name, use similar logos/favicons on the sites themselves, and keep them connected, even though they're separate places with their own themes/content.

I'm not sure how much sense that made, but for example, Site A might be Funny Dogs and Site B might be Funny Cats....so I'd use an animal favicon, go by something like "Animal Lover," etc, and link to the opposite sites on one another.

Are there downsides to keeping the sites connected and somewhat unified in this way? Would it be better to create unique identities and promote separately all the time?

This would depend on the sites. In some cases it is better to separate the sites in other cases it is better to keep them together.

With your example I think it would be fine to have a general identity and promote the opposite site since most people would consider themselves more of a dog or cat lover and having your identity match wouldn't be as relatable when promoting an opposite site. Its not trustworthy for a cat lover to push a dog site, you know?

Otherwise it may or may not work. If the two are connected but slightly different a more specific identity might work better, but having an easy to identify identity across many sites could be better.

I think that using the same and sucesful technology is good way to get other web site sucessful. I think that it is very good idea.

Since all your sites are of similar genre I think it would be better if you keep it remain connected, however you may consider to do a generic benchmarking from each others and as well from top performing animal sites appearing on top search results.

I think that the only way that it would prove particularly detrimental for you is if the one site got a bad rep. At that point you may wish to distance them.

Otherwise being able to pick up visitors potentially from the other site can only be good.