Best way to simultaneously control two servos positioned horizontally and vertically?
What kind of servos do you have?

As far as i know there ar analog servos which you can drive with a pwm. Every microcontroller i have seen has some pwm outputs. With them you can generate the signal and it runs in the background without any processor resources needed.

When you then stepwise change the ontime period of one pwm the other one should rest as it was. So no multi threading is needed in this case and solving your problem is really easy and straighforward.