I would guess this is going to damage your boards in the long run. I am no electrical engineer, but as far as I know, the inputs of electrical gear have high impedance while outputs have low impedance. When you connect the two keyboard through a Y cable you are in effect connecting the outputs of the keyboards as well; that is you are connecting two low impedance terminals. So a signal that appears at the output of one keyboard can easily intrude the other due to the low impedance and can damage it in the long run.

Again, I am no expert, but you will be at the safe side using an inexpensive mixer. Have you tried Behringer mixers? They are pretty good for that price.

Shiral