I uses to use a portable power pack frequently (my cell's battery wasn't really sufficient for a whole day) and I still use them when I'm traveling.
I'd think about the mechanics of charging two phones at once. You'll have the battery pack and some usb cables, plus the phones to keep all together. A charge from 10%-100% is going to be 2-3 hours at 1A and I find that mighty inconvenient - especially if I'm the one who handed my phone off to be charged. We carry a charger each, or pass a set of shared chargers around as needed.
Make sure you get a battery pack with sufficient capacity. Your phones have a 2100 mAh battery, so you'd want a pack with at least 6000 mAh to charge both phones.
Our travel chargers have lasted through 3 phone upgrades, so I'd probably buy something that could charge at 2A, and has the largest capacity I'm willing to haul around/pay for.