Lots of great advice here. Could've done with forums like this a few years ago before I made my mistakes!.....
I've usually had more than one dog at a time, but won't say it's been without it's problems. I had Sabre when Max was 10, and they never got on from the beginning! Max just didn't want to know. They never played with each other, and only tolerated each other because I made them! We did have a couple of fights, which weren't nice.
When Sabre was 3, I had Misty, GSD pup. She was very much like Keira when it came to energy, she was a real live wire! I realise now that I had her at the wrong time, I'd changed roles at work and began working silly hours, so didn't have the time to commit to them on an individual basis. As she got older she became more hyper, which was too much for Sabre because he's quite a laid back dog at home. She wouldn't leave him alone and I had to separate them because she was basically terrorising him most of the time. In the end I re-homed her, much to my regret, but it was for the best for both of them, and I definitely learned from this experience! I'd never have another dog now without being able to commit the time needed to both dogs. I also had my sons dog come to stay for a while, 2 year old GSD... and that was a nightmare! But it's not all bad I have had more than one dog in the past without problems, so don't be put off. Just giving you my worst experiences!
My mom's 2 dogs, who have had no formal training, absolutely adore each other. They play, groom each other and sleep snuggled up together... but they are yorkies! And both are still very much attached to my mom. So I think it's about having time and commitment, and trying to chose a new dog with a temperament and won't clash with with your existing dog. And like farah says your bond with both dogs is really important. By spending time and doing training with Sabre our bond is very close, and yours with Keira will become deeper through your training with her. Unfortunately I failed Misty in this respect, which is why things didn't work out.
So that's my past experiences. Now Max has gone I know in time I will look for another dog, but am also thinking of giving fostering a go. Sabre is fine with other dogs coming into our home, so may trial this out once I've moved house and see how it goes. I'm lucky that now I have time to commit to another dog/dogs, and I won't be making the same mistakes in the future! With you not working at the moment it's an ideal time for you to introduce another dog into your family, you just need to decide whether it's an ideal time for Keira? Think you'll get a feeling when the time is right, so listen to head as well as your heart (which is often something I fail to do!!) Whatever, your heading in the right direction with Keira's training. And if things are a little bumpy at first I know you are a committed owner that will work hard at dealing with any issues.
Wish you luck hun with whatever and whenever you decide.... and if you decide to rescue I know of loads :)