If you are using MPLab just have something like this:
pwm01 = PWM_in2; // Left drive = left joystick up/down
pwm02 = PWM_in3; // Right Drive = right joystick up/down
//for one motor on each joystick left/right
pwm03 = PWM_in1; //extra motor 1 = Left joystick left/right
pwm04 = PWM_in4; //extra motor 2 = right joystick left/right
//for multiple motors on each joystick left/right
pwm03 = pwm04 = ... = PWM_in1;
pwm05 = pwm10 = ... = PWM_in4;
I don't know EasyC
, but you should be able to do something similar.