When we first took a look at installing the SteamOS beta, it lacked important features like dual-booting and using non-NVIDIA graphics cards. The latter was solved earlier this month, and now dual-booting has arrived as well.
The new SteamOS download now includes the ability to create dual-boot options as well as custom partitions. It does come with a big warning that these features have not yet been tested thoroughly. As with anything labelled beta (and not made by Google), you should be extremely careful when installing it, and avoid doing so on a machine you depend on.