Hi Jack! Welcome to YT! I just have to echo advice you already have. Potty train one at a time. Otherwise you never know who did what.
But once she is trained, another is a great idea. I love having more than one! They love to play together. It took my newest girl, Annie all of 2 weeks to break the barriers of being the new girl. Now my other two play with her like she has always been here. She was persistent.
Also -- being 1.2 lbs at 14 weeks may or may not mean she is going to be very tiny. As you have already read, those early weights can be misleading. So, you won't know for sure how large she is going to be until a year old. Even then, some have reported a growth spurt after a year, although those are not common. My girl Annie was 1.3 lb at 12 weeks but she is really gaining now so I won't be surprised if she winds up 4 pounds or more. I read of another that was average weight until about 5 months and then she just stopped growing and wound up a tiny. Adults size is one of the surprises our Yorkie pups have in store for us. BTW my little yorkie pup of less than 2 pounds, my 4 pound adult yorkie male and my 11 pound Jack Russell all play extremely well together. In fact, the 11 pound JR is the most gentle of the lot!
On the barking -- she will find that voice in her own time. I always get a kick out of their first ferocious little barks. Annie just brked last week for the first time and now she joins in after the bigger dogs bark even though she has no idea why.