Remarkably, the Outlook Rules Wizard does not support OR'ing in the way you've described. You can create two separate rules to get the same result, if you don't mind a proliferation of rules. The only other alternative is to create a custom rule in VBA script, but it is not a trivial task by any means -- I wouldn't recommend it. (If you are interested, you might peruse the links found in this question.)
UPDATE: I should mention that it is possible to combine multiple source emails in a single rule ("or"-ing them together, so to speak). This doesn't answer the original question, but it is something I just recently realized is possible. To do it, simply find the "from" field in the Rule Description window, and click on the hyperlink, then manually edit the "From" address line at the bottom, adding as many addresses as you want, separated with a semi-colon.