If a browser doesn't support responsive stylesheets they will simply ignore any of the styles inside of the media query. For this reason, you can simply include them together in the same stylesheet. If you are doing this for optimization purposes I would consider using server side scripting to parse the user-agent
string and then append the stylesheets necessary. You could also use the Internet Explorer conditionals to add the stylesheets using the <!-- [if lt IE9] -->
conditional.
There is also a polyfill for unsupported broswers called Respond.js. It will parse the media queries and add the styles as-needed in older browsers at the various sizes.