I would suggest you look at htcs own battery bar if cost is no object. It's 6000 mah and about fifty quid. There are cheaper ones out there for half the price, but you can be sure they won't be the output they advertise and will die more quickly.
There are a couple of good ones on Shopandroid.com
Personally, I use THIS one. It has two 2A outputs, with a built-in cable so you don't have to carry one with you if you don't want to. Yes, it'll charge two devices at the same time. I've charged my One and Nexus 7 simultaneously with no issues.