Yes, you may use a Y harness but it is not the optimum way to do things. As stated by the others, you will have a lot of mechanical work to do to set up the plane right (getting the correct differential, the same throws each side, etc). And each time you have to do a minor adjustment, you will have to do it mechanically. In other words, as long as your radio has a flaperon functio, there is no reason to use a y harness. What radio do you have?