That's where the business rule comes in. But they both have to be completed before the remaining activities can begin. Make a transition from Receive request to the fork. Which you choose depends only on how you want the diagram to look. Be sure to look for the halo when you draw a transition to a fork.
Forks are so slim that it is easy to miss a target fork and land on a swimlane instead.
If you try to end a transition on a diagram entity that is not a valid target, Together writes a red error message in the Message pane, and it displays an error box like the one below. The join button is the same as the fork button. You can choose either the horizontal or vertical version. Again, which you choose depends only on how you want the diagram to look. Make a decision node to compare the number of tickets to the capacity of the airplane.