I'm trying to use two servos simultaneously and I've connected both servos to my battery pack which gives 6V (the servos require 6v). If I ground the servo only to the battery and I only use one of them it works fine. If I want to use both, the servos just stop and one of them sometimes moves a little bit. I noticed that if I wire it also to the ground of the RPi (with a 1k Ohm resistor between) it works fine. I can use both of them simultaneously. So why do I need two groundings to work fine? Can't the single grounding handle both of the PWM outputs from the Pi or something in that direction?
You need to create a circuit for electricity to flow from the Pi to the servos and then back to the PI.
You control servos by sending 1-2 ms pulses along the control wire at 50-60 Hz.
If you don't have a ground wire between the servos and the Pi there is no circuit for the pulses.