Having your projects separate is a neater solution and less prone to errors BUT there is absolutely no reason why you can't have both languages in the same project. If you adopt the later approach it would require some use of conditional build tags to exclude output or set-up the DUCC feature that came in with RH9. You would also need a set of files for each language (e.g. TOC, Index, Glossary, Mapids) and ensure you use the correct ones for each language. The best way to achieve this would be to set-up two single source layouts each with the required properties to pull out the required output.
So you can see that using one project is possible but there are a number of considerations. The typical approach is to author in one language, create a copy it, send that off for translation and generate the output.
Ben Minson wrote a DevNet article that may help. http://www.adobe.com/devnet-archive/robohelp/articles/localized_projects.html
Until Rh10, you really needed separate files for each language as whilst the content could be tagged, the Topic Title would be in whatever language you entered it in. Hence Colum advising you need a separate set of files for each language, be they in one project or multiple projects.
Rh10 allows you to use variables for Topic Titles so you can create a variable for each topic title and then enter values for each language.
See www.grainge.org for RoboHelp and Authoring tips