If you use a Y-harness it will have to be the reversing type. These have a circuit on one branch that reverses the signal and causes the servos to rotate opposite of each other. If you look at the way the servos mount they will move the elevator halves in opposite directions if the servos rotate the same way. The reversing Y was made for this.

If you have a DX-7 though I'd just plug the second elevator servo into aux-2 and mix them. That way you can reverse them individually as needed. Be sure to use programmable mix 5 or 6 so the elevator trim will work for both servos. With all the other programable mixes the master trim will not affect the slave.