how can i Attach both hands to the same object (gun's main grip and front grip)? i already attached the right hand's bone to the weapon's main grip (using select weapon > ctrl+p > to bone ), 

now i want the left hand (with the same holding position) to keep automatically following the weapon's front grip. what should i do ?
You have to make a seperate bones and on the wrist use the copy transforms constraint. Place the Seperate bone at the foregrip , apply the constraint to the wrist and the should follow , You will need to tweak the position of the bone , but it will work.