Karl,
You should be able to do this easily at your client side (test side) using your TestRunner. You don't need Selenium Grid to deal with this for you.
The benefit of doing this at your client side, is that you now no longer have to worry about which node the test is being routed to (which of-course is an internal behavior of the Grid Hub )
For example, assuming that you are using TestNG, you could basically create two <test> tags each which runs with a different URL and the URL is passed as a parameter to the <test>
TestNG lets you build dynamic suites, so you don't need to worry about having to duplicate the <test> tag many times one per URL.
Thanks & Regards
Krishnan Mahadevan
"All the desirable things in life are either illegal, expensive, fattening or in love with someone else!"
My Scribblings @ http://wakened-cognition.blogspot.com/