One solution I've had for this is to let one of them use my iPod Touch while I work with the other. I've downloaded a few educational games for them so that this time is productive, and so far it has made it easier to focus on one child without having the other lose time that could be spend practicing needed skills.
I'm struggling with good apps for my 3rd grader though. I want to keep it to the free apps, so I guess for now it'll be trial and error until we find the ones that work best for us.
Have you found any great educational apps? I'd love to hear about them (especially if they are free!).