Ignoring the fact that it seems to mangle my model after the side profile, it says in the Joystick Controller README:
[…] You can create multiple sliders and combine different poses.
and
…Similar to 5, 7 frame. But the power of slider is that you can combine whatever the value of 3 and 5 frame is.
So I’m curious because I have no idea what that last part means.. does that mean I can combine all these sliders? And if so, how?
@veermetri05 is the author of the plugin, maybe he can help 
But whoever knows, please help him. 
There is a long forum thread here about his plugin: Joysticks in Synfig (Controller Plugin) .
Joysticks consider 5 waypoints as a default (neutral/center) position and Right, Left, Top, Down.
Running the script calculates all the min/max and moving the joystick interpolates.
For the sliders, it is a bit different, there is always the first pose/waypoint as a start.
Then, for each other waypoint, it is the end of an animation.
And for each of them, there will be a slider, using the defined waypoint as the end of the animation.
You can combine them.
For example, think about a first waypoint as a closed fist.
Then you do other waypoints where each of them is an extended finger.
Each slider will animate from the closed fist to a specific extended finger, independently.
You can do the same with closed and both opened eyes.
One waypoint/slider per eye, so you can animated them independentely, with intermediate (tweening) value, instead of using on/off switch layer.
Note: it is not compatible with what you expect to do
@veermetri05 If I may suggest an approach, you could implement ValueAtTime using the Time Loop converter. This allows the data to be stored as waypoints, with values retrieved at a specific time (t) through a controller.
You can take a look at how it works in this file:
play_mode.sif (578.0 KB)
input_mode.sif (103.6 KB)
@Slam , for the usecase, using a joystick is more suitable. And to answer your question, yes you should be able to combine multiple sliders. What it means is : You have slider1, slider2, now you can create slider3 which controls slider1, slider2.
I will attach a small demo, to showcase how I can use joystick for such head turning animation.
@zainal , I will take a look into it. Ideally sliders and joystick should control anyway waypoints whose value can be interpolated, so I will take a look at it.