Since you checked this selenium-rc, I assume you are using rc.
Just use two separate instances of selenium in the test client. There is no reason why you should have only one instance of selenium within the same class. In my opinion, this is even cleaner as you are talking with two different user interfaces.
On the server side, you still only need one. The server will happily launch two instances of the browser if asked to use two instances of the client.
Moritz
edit: just to add this, selenium can now be instantiated anytime with
Selenium s = new DefaultSelenium("localhost", 4444, "*chrome", "http://your-target-url");
Assuming the selenium server is on local port 4444.
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