i love this ship. yes they are soulmates no they have never spoken in cannon.

After Tempest turned her back on the Storm King and abandoned her allegiance to the Storm Army, the government of Equestria were unsure of what to do with her. Celestia's court of highborn ponies called for imprisonment or even banishment as punishment for her crimes, while the Crystal empire requested that she be released to them and made into a proper soldier of Equestria, but in the end Twilight stepped in and suggested a new path for Tempest. She suggested redemption. Tempest expressed a desire to live a life of comfort after so many years in the army, and maybe use her newfound special talent, fireworks shows, to good use. Twilight was ecstatic and more than willing to help her new friend try new things. She asked the cutie mark crusaders to take Tempest to parties and festivals and show her how happy her talent could make others. While Tempest enjoyed this, the constant public appearances and pressure to be the best left her tired and a bit unhappy. She vented her frustrations to Luna when the dark mare appeared in Tempest's dreams to keep the nightmares away.
Luna, a pony who also avoided the constant bustle of modern life, sympathized with Tempest. She offered her a place in Luna's inner circle, as a lady in waiting, and as a friend. Soon Tempest was living in the castle in Canterlot and having a daily tea-time with the introverted princess. They became fast friends, with both mares being inexperienced in the ways of modern Equestria. Together they went to the theater, enjoyed picnics in the moonlight, and tried the finest dining the kingdom had to offer. Wait. Were these dates?
Tempest felt like an enormous idiot upon realizing that she was being courted. Nobody could deny Luna's beauty and allure, but she was shocked that Luna was so casual about her feelings. After this realization, Tempest was hyper-aware of how Luna treated her, noticing the small gestures like preparing Tempest's tea just how she liked it without being asked, or how the dark mare would often visit Tempest's dreams just to spend more time together. Tempest was beyond flattered and honestly quite flustered. She was very inexperienced with relationships, and found herself flushed and excited every time she was near her friend.
Things did not come to a head until Luna wandered into a dream of Tempest's. One of romance and passion. Tempest was mortified when Luna walked in on her necking with an imaginary version of the princess. Luna thought it was cute. When morning came and they faced each other in the waking world, Tempest tried to apologize, but Luna stopped her and asked for her to never stop having dreams like that. A proper debut as a couple followed soon after, and a wedding not long after that. Today Luna and Tempest live in quiet domestic bliss, and raise their two children: Zodiac and Midnight Dream. Luna tries not to think about the mortality of her wife and son.