Voltage drop, current increase when two servos connected
The problem is simply that you have too much load for your batteries.
Battery voltage drops when you draw too much current (when the load is too heavy.)
Boosting the voltage draws more current from  the battery, making the battery voltage drop even more.
Every voltage booster you add just makes the problem worse.
You need a battery that can deliver more current. More 18650 cells in parallel, or just a bigger battery.
The voltage to the Arduino can be lower, as well.  You are boosting 3.7V to 10V, just to throw power away in reducing the 10V back down to 5V on the Arduino.
Boost the battery voltage to 5V, and power the Arduino over the 5V pin.