Just copy the Rome folder, paste it, and rename it to whatever you want. I used to have "RTW 1.5" as backup, a "RTW - EB" and "RTW - Arthurian" folders for instance. It's not a necessary step, but saves the hassle of reinstalling Rome should you tire of EB or want to install another mod, or even just keep playing regular RTW. All you need to do is copy the RTW folder again for another mod.