Tips for reference .
1.You may check if the height of x-axis are the same on both sides .
If not ,you may turn off the machine and remove Z-axis timing belt ,then twist the Z-coulping to adjust it .
2.When leveling central point of Extruder 1 , you may adjust the Z-limit switch .then adjust the rest of four corners by twisting the bolt.
3.When adjusting Extruder 2, you may loose the four screws and then adjust the height of extruder 2.
Here is the video that shows you how to.