depends. the truth is everyone will tell you something different. honestly, i did this with great success. however.... if you train your chest with DB's and try training your triceps the next day, you're in for a world of hurt (at least if you keep the DB's close to your body). the real factor here is : are they sore after a chest workout or the next day before your arm workout? if yes, then its bad to train them. if not, go ahead and rip them apart.