i had that problem and then one day i could just do both. for me all it took was practice. some songs are easier to sing and play at the same time. today by the smashing pumpkins is one of the first ones i learned to sing and play on. some silverchair songs, on the other hand, are hard as **** sing and play on. anyways my main point as simple as it is, is that it is just a matter of practice, and understanding the rhythms