How to have 2 installations of San Andreas?

I have a copy of San Andreas installed on my computer. It has a REALLY good modification installed, called Things To Do In San Andreas. (Check it out if you have SA), anyway, I also really want to play San Andreas Multiplayer and Multi Theft Auto however this mod adds a lot of new missions, and things so it modifies the games files. Therefore I cannot play SA:MP and MTA, how can I have Two Installations of san andreas, so one clean and one modified?