Consider the following variation of the standard Turing machine model. There are two tape heads which are exactly next to each-other pointing to adjacent cells. The heads move simultaneously two positions each time, either to the left or to the right. Each transition is of the form (x, y)(u, v), L or (x, y)(u, v), R, where symbols x and y are in adjacent cells, and replaced by symbols u and v, respectively. Prove that this model is equivalent to the standard Turing machine model.