Variance / Covariance question

I read a little bit on SW Professional BJ regarding playing 2 hands versus 1. Not sure why you have to bet more on 2 versus 1 hand to keep it the same but if someone could elaborate more, that would be great. Does your edge go up higher with 2 hands versus 1? Do any of you play 2 versus 1 hand? If so, does your edge go up much more? If so, how much more?
Thanks,
Brendan