You fuse your thoughts with an ally’s, allowing the two of you to fight in tandem, perfectly coordinated. You and the ally each roll initiative in combat and use the higher die result before adding modifiers. This has three effects.
Spell: If you both cast spells and target the same area or same creature, affected creatures take a –2 penalty on their saving throw against the spells.
Source Pathfinder RPG Ultimate Magic
When you cast this spell, if your linked ally uses a mythic path ability against a creature, you can use the same path ability against that creature as if you had it yourself, and your linked ally can likewise use any path abilities you use against a creature (even if that ally isn't mythic). For example, if you're a champion attacking a frost giant using the precision path ability, your linked ally can use your precision ability when she attacks that frost giant. If a path ability requires expending uses of mythic power or any other cost, your linked ally must expend that cost for you to use the ability.