"In the long term, you probably won't want to keep them together anyway since it means you will almost definitely have "surprise" batches of eggs, and breeding (or attempts at breeding) is really hard on both axolotls, especially the female. If they are kept together, she especially will likely develop a weakened immune system and a general health decline.
I would suggest doing lots of research into the whole breeding process (from getting them to mate, and then egg, hatchling, and juvenile care, to rehoming the babies) and make sure you have a definite plan before making any big decisions"
This is the same bs i see in moronic fb groups. Its utter nonsense, axolotl do not kill themselves with over breeding nor do males and females need to be kept seperate. Its highly unusual for females to lay to an extent that it could damage their health. Take your own advice and do your research in places other than fb and you may stop peddling this crap.
To answer the ops question, axolotl arnt territorial , as long as they are the same size they should be ok, just put the new one in the tank and keep them well fed.