possibly zangetsu didn't tell ichigo because he is supposed to find out on his own. besides when ichigo was training for bankai zangetsu did show another power, the whole splitting in to multiple bodies. tho arguably it could be said that is just fully mastering the speed part of the bankai.
Originally Posted by shinobu03
if thou art looking for me thou needest look no further than the deepest recesses of thine own pitted, black soul. if i am not there call hell, i'll be there playing poker with satan over ownership of hell