You could just put both versions in a single .zip file together, so all buyers would download both versions together in that file and use whichever one they need (or both, if they have both Mac and Windows computers).
If you'd prefer to sell them separately, then you would set up a separate product in Seller Admin for each file. You could also offer a "package deal" on the purchase of both versions by setting up a third product using "Package files from other products"; see Approach B here:
http://www.e-junkie.com/ej/help.package.htm#bundle
