WHen you are doing arms, jsut concentrate more on pulling/pushing harder with your weaker arm. GIve it more "attention". Like on barbell curls, dont curl the wieght up unevenly, make sure you pull hard enough with the weaker arm so that the bar is straight and even, even if that means letting up a bit with the streong arm. Also, at teh end of your workout, add in 2-3 sets for your weaker arm. I would not give it a seperate full day though.