The only folder RTR, or any other mods in this matter, change is the Data folder. A huge folder, in fact, but just that, and not the entire Rome directory.

All you need to do is copy it. In fact, just click at your data folder, click copy, then your computer will name this new identical folder in the same Rome directory as "copy of Data." Just leave it there knowing that's original 1.2 Rome and install RTR. RTR will install into your folder named "Data" (only, without the quotes.) When you want to multiplay or play original game, just change the name of the current "Data" (RTR one) into something like "RTR Data" and rename the "copy of Data" folder into just "Data."

Switch at your leisure. No need to even move the data files to a seperate directory somewhere.