Great idea!
I just made a short experiment and it seems to work fine. The touch driver on the STM32F746 Discovery board provides everything that is needed. You can enhance the function GetTouchPosition() within the file BoardConfig.c in the following manner:
Add a second point parameter to the function GetTouchPosition(), so that it looks like
int GetTouchPosition( XPoint* aPos1, XPoint* aPos2 );
Within the function you should evaluate the content of touchPadState.touchDetected - it contains the number of detected fingers.
The position of the first finger is stored in touchPadState.touchX[ 0 ] and touchPadState.touchY[ 0 ]. This should be copied into the given parameter aPos1.
The position of the second finger is stored in touchPadState.touchX[ 1 ] and touchPadState.touchY[ 1 ]. This should be copied into the given parameter aPos2.
The return value of the function is the number of detected fingers.
Within your main loop, you can now use the return value and the position data of the function GetTouchPosition() to feed your UI application.