Compared to other devices, there is no "Move to SD card" option in the application manager menu on Galaxy S Duos. I have rooted my phone and I use folder mount app to link game data to SD card. I have tried and tested this app, so I can guarantee that this will work on a rooted S Duos. But first, you have to install the game in internal storage. Games like "NFS Most Wanted" are more than 1.8GB, i.e. it exceeds the internal storage. So,in order to play them, you need an app called external2internal which swaps internal storage to external. But these apps will not move the whole game. It only moves the game data, but not the APK.
You could find custom ROMs with "Move to SD card" option for this device. But again, the ROMs take away many features while giving some. I recommend NOT to flash any kind of ROMs, and those new users who don't know how complicated rooting a device, I will recommend them to use Kingo Android Root to root their device. But be cautioned, it will void the warranty. In conclusion, S Duos users who are not rooted have no practical solution for this problem and are left helpless.