On a somewhat frequent basis, I am asked about the use of multiple websites in a search engine marketing strategy. 'Is it a good thing? Is it a bad thing?' The answer is, it depends.

It depends on what you want to accomplish and it depends upon your current stage of web marketing. If you already have a website with strong overall positioning that covers all of your services, then a secondary website may be worth considering. If on the other hand, you're just embarking on a search engine optimization campaign for the first time, I wouldn't recommend starting off with multiple websites.
Usually, when we talk about a secondary website, it is a focused "mini-site" that deals with only one area of your practice. We have a
plastic surgeon client in Philadelphia that has two mini-sites: one for
breast augmentation, and one for
body contouring. They do very well, but it took a lot of work.
Secondary websites require the same search engine marketing as your main website because the same search engine evaluation rules apply. From the search engines' point of view, there is
no affiliation between your main site and the secondary site whatsoever, and every website must succeed based on its own merits. You can't expect your second website to perform well just because your main site is well-recognized by Google.
The strength of a mini-site is that it provides focused information. Google and the other search engines really appreciate "authority websites" that have specialized information on a given topic. So, a 15 page mini-site that deals exclusively with brain injury stands a better chance of obtaining 1st-page listings than a website that deals with 30 areas of practice with only 1 or 2 pages devoted to brain injury.
If you choose to do any linking between your multiple websites, the links should be one-way only. You must decide whether your secondary site will exist to support your main site, or if you want your secondary website to be a dominant presence unto itself. In the latter scenario, it is best to link from a relevant internal page of your site to the secondary site. Do not link back and forth between the websites or Google and the other search engines will recognize that the two sites are in cahoots and the links will cancel eachother out.
A mini-site is a good investment if you're looking for a way to take your search engine marketing to the next level. You can then achieve 2, 3, or even 4 listings on the first page of the search engines if you have multiple sites. Combine that with a pay-per-click campaign, and you can increase your exposure even more. Just understand that you may not see the same level of return on investment from your secondary site that you've seen with your main website.