We tried the leash for two when we had two small dogs and found that until they are trained to walk properly with you, they will tumble and romp over each other and get the leashes all tangles up. It's easier with two separate leashes, not too long.
The main thing with training is that vigilance and lots of praise works, but they're very young, so it won't be accomplished overnight. It might be more confusing for them to be together during the training episodes, at least until they know their names, so if you have someone there who can take one while you take the other, then the praise when one goes will not be heard by the other dog as just a random human noise. Always use their names when you praise them.
I don't like the idea of newspaper training because it doesn't teach them anything about going outside, and there's always the possibility, at least around our house, that someone will leave a paper on the floor that wasn't intended for that purpose. I think you're on the right track with crate training. My brother has two chihuahuas who consider their crates to be their sanctuaries. They go in voluntarily. They have a long ride between their home and their new grandson, and the little dogs are very good about riding in their crates, too.
If you haven't visited chihuahua sites online, that would be a good place to get helpful ideas and more information about what this breed is like to train. Here's one I just googled up (not an endorsement, since I've never seen it before):
http://www.chihuahua-people.com/
One other thing, hydrogen peroxide is the best and cheapest thing to use to clean pet mistakes on carpet. Test first in a closet or other hidden spot, but it will probably be fine. It saved us a bundle when we had three elderly dogs who became incontinent near the end of their lives.
I have not had a chihuahua, but other people I've known who did always seemed to adore these little dogs. Yours are awfully cute.