Adjust touch parameters
This is a hardware related problem. The touchscreen sends your system a touch event for every finger you touch it with. If the touchscreen's touch resolution is not that high, it will merge two touches to one.

You can't change the touch resolution of your touchscreen but it is possible to send custom touch events to the Android framework using some commands (I don't know them now but I used them and it is working. It is also similar to Windows' event system.)