With a Y connector, both servos are moving in the same direction - when you have one on each side of a plane as you do on your Yak, one elevator goes up and one goes down. To use a Y connector and make it work, you will need a Y connector that has a servo reverser built in - MPI makes one. I prefer to run each elevator half to a separate channel and mix them with the TX - and reverse one servo within the TX programming. I am not familiar with your radio - check it out - that may be the easist solution. There are programmable digitial servos, but I am assuming you are not using those.

Dan