The 74HC04 (high speed CMOS) IC is good only up to 5 volts dc input. Higher than that and it may start to oscillate.

The 7404 (TTL) is good for up to 7 or 8 volts, if you really think you need to use an electronic Y.

Also, make sure the unused inputs are tied either high or low, or they too will jump all over the place.

Best bet is to hard wire the 2 servos together, no IC, no chokes.