Installing applications with larger size on S DUOS I am using Galaxy-S-Duos 
Android 4.0
Internal memory 1.8 GB !

What if I want to install application like Modern Combat-4 (1.9 GB)

These applications require to be installed in phone memory for their 1st run.
After that the can be moved to SD card..

Is it possible to install them directly to SD card?
Installing apps from Google Play will first download the .apk file to /data/local, which is in the device's internal memory – so you're already lost before the installation itself even had begun. And that's only the first hurdle to be taken:

Though one could set the "default installation location" (using adb set-install-location, see adb help), the app must explicitly support to be installed on anything other than internal storage. If it does not support this, it cannot be installed there (though you might be able to move it there lateron with some special apps enforcing it).