Not sure what you mean by that. There some websites that just use Flash for the cam, and you can stream one cam to two sites since it's the same plugin. If you're referring to sites like Twitch and Justin.tv, not sure there is one software program that can do that on the Mac side. OBS and XSplit on the Windows side can do that with significant tweaking to settings.
The tricky part is streaming to two sites at once. That's where apps run into issues. The multiple browsers thing works for sites using the Flash plugin, but when you have to use a separate app it gets more complex.