If you mean you have two apps displayed on the screen then there are 2 'modes', if it's 2 thirds one third then simply tap on the two thirds portion and it should revert to full screen. If it's half and half then there should be a black line down the centre with a grey bit in the middle, put your finger on the grey bit and drag it to the side of the screen (right or left) and again it should revert to full screen. If you mean that you've managed to split the keyboard in two then touch and hold the keyboard down key (the one with a picture of the keyboard on) and an option to merge and dock will appear, you need to slide your finger across to select it without lifting off or it disappears.