Disable the other dropdown by javascript if the user makes a selection from one of the dropdowns
when the page loads both options will again be available until one of the dropdowns is used
...you can then happily use OR in the select.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.