i have cut my cement board too short for my shower walls
I'm going to speculate that you've set your cement board tight to the floor, and that it reaches two inches short of the height you'd like your tile to reach. I'm also going to guess that you haven't installed the drywall above the cement board. 

Yes, you can bring your drywall down to the height of the cement board and lap the tile onto it. In fact, this is a more ideal scenario since it leaves you with a clean surface on which to terminate the tile. You'll probably want to finish the drywall and paint before setting the tile so that the grout or trim can lap over the paint for the cleanest lines.