Changing text constantly

Could someone help me with this as i couldnt think of a way for this so i have 2 textrenders A1 B1 and what i have is a door and a cube and text rednders are within the cube so what i want is a way to make a1 b1 keep showing up on the cube in the game as for example a1 b1 a1 b1 a1 b1 constantly