Glad you got it working :-) To connect another pad you just need to add another line to /etc/rc.local (straight after the other line you added):
xboxdrv --trigger-as-button --wid 1 --led 3 --deadzone 4000 --silent & sleep 1

Info from the RetroPie Wiki: https://github.com/retropie/RetroPie-Setup/wiki/Setting-up-the-XBox360-controller

Then you will need to add 2 player controls to your Retroarch.cfg, just copy the player 1 controls and paste underneath. Then change input_player1_joypad_index = "0"
to input_player2_joypad_index = "1"

All keymaps need to change to input_player2 instead on input_player1, for example input_player2_b_btn = "1" etc