How do I add a random string to a URL to prevent caching if a question mark already exists?

I need to launch a random URL through Flex in order to prevent caching.

The current URL ends in /?propertyID=Neighborhood1&planCode=StreetScene

I can usually add a + ("?"+Math.random().toString()); to make it random, but that doesn't work at all in this case.

Is that because a question mark already exists? What is the best way to append a random string to override caching in this case? Thanks for any help