There isn't really much support for multiclass characters in the playtest; the closest being the Dual Path Mythic feat. However multiclssing is not required for that feat. Pathfinder itself does not really encourage multiclassing even though that is the only way to build certain character concepts.
In support of multiclass characters I propose the following Mythic feat: -
Multi-Path (Mythic)
You excel in many areas, allowing you to select abilities from several mythic paths.
Prerequisite: 1st mythic tier, Levels in 3 or more core classes.
Benefit: Select a number of mythic paths, including the path you selected at your moment of ascension (see page 3), up to the number of core classes that you have. You gain the 1st level ability of each path selected (archmage arcana, champion strike, divine surge, guardian’s call,marshal’s order, or trickster attack). The path you selected at ascension also gives other abilities such as bonus hp etc. as normal. Whenever you gain a path ability, you can select from list of abilities presented for all selected paths, as well as from the list of universal path abilities. You may also choose your trials from all those appropriate for all your selected paths as well as universal trials.
Special: Should you reach the 10th mythic tier you also gain the capstone ability of each path you have selected (true archmage, legendary champion, true defender, divine vessel, visionary commander, and/or supreme trickster).
Special: If you should acquire additional core classes after selecting this feat you may gain an additional path for each additional core class.
