How to add a ceiling light to existing ceiling light in same room that has two wall switches?
There will be a pair of conductors coming into the light box, one of which is the switched hot. You can tie into those to add another light either in the light box or in the switch box at the other end of that cable. 

Be sure you have an understanding of box-fill regulations, connection techniques, etc. before you make any changes. For further assistance, show a diagram of the current situation.