You can check with US, but I'm pretty sure they will only allow you to put one pet reservation tacked onto yours. They will let one person carry on 2 cabin pets, but they have to buy 2 seats in order to do it. Since you're travelling with a group it would probably be easier and much cheaper to put the other pet reservation onto one of their tickets. I think the current price for an in cabin pet is $100 each way.
You are allowed one carry on and one personal item. Personal item is defined as a purse, briefcase, computer case, small backpack etc. If your purse is small enough you can pack it into your regular carry on and then have the pet as your personal item since the pet carriers aren't that big. I carry a pretty big purse (as do many of the women going on trips) and there's no way it would fit into my suitcase.