I think it takes a lot of brainpower to do both things at once. I think you kinda have to get the guitar part wired before you can even do any singing. So it's like the guitar is on autopilot and you're singing with all ur concentration. If that makes any sense. I can kinda do it when I play Babe I'm gonna leave u by Led zeppelin. I've played that song a million times. I have no idea how guys like Yngwie, Satch, Vai, Clapton and Hendrix ever got around to learning hard guitar parts and vocals. Smart guys...