A new Season of Discovery questline has been discovered and reported to us by Wowhead user Geartrix, allowing Goblin and Gnomish Engineers to learn recipes from the other specialization!
Goblin and Gnomish Engineers have traditionally been blocked from learning recipes associated with the other Specialization once locked in. This, however, looks to have changed in Phase 3 with the implementation of Engineering Exchange Ticket.
After leveling Engineering to 225 and unlocking a Specialization, players will have access to a new quest (Gnomish Solutions/Goblin Solutions) offered by Fizzbizz Coppercoil in the Shimmering Flats in Thousand Needles.
For Gnomish Engineers, this quest requires two Gnomish Harm Prevention Belt, while Goblins need to provide 5 Goblin Land Mine. Turning in this quest gives you another — this time to kill Thunderstomp Stegodon in Un'goro Crater for a Thunderstomp Stegodon Horn (Require Gnomish Problems/Require Goblin Problems).
Next, you'll need to complete the quest Watering Down the Competition by heading to Hearthglen and finding a Box of Scarlet Dye upstairs in one of the houses next to the bed. Return to the Shimmering Flats once more, and after a few more quests such as Victory Lap for the Gnomes!, you'll be directed to speak with Maximillian «The Greaser» — who will offer the quest Hush Money, which can be completed weekly for an Engineering Exchange Ticket.