I've actually found from experience it's far easier to not thing about doing both things at the same time, and do it when it feels natural. Don't FORCE yourself to sing and play at the same time, rather practice in little pieces. What I mean is, play a portion of the riff (a small piece) and try to slowly sync your voice with it.

It's easier than playing it full speed and going for the kill while you really don't have much practice, which would lead to infinite frustration.