Try a burnout with the wheel pointing to the left a little or on a left hand turn and see if you can get both spinning. I think it favors the right by default.
before doing burnout put car in reverse back up a little then put in drive or 1st gear whichever put foot on brake give gas there you go two wheels will spin.our cars dont have posi rears we have limited slip dif....now if you want them both to spin you need to install a posi rear!!!!!!