It's likely your occlusion that decides whether it's the masseters, the temporalis, or both that get activated. Try to move your jaw back and bite backwards (be careful with this if you're at risk for TMJ), and then jut your jaw forward and bite forwards. You may also try biting side to side. Do you notice a difference in what muscles are activated?

As Elwynn says masticory activation is based on how forward your mandible is related to your maxilla. a maxilla that is set back usually is more temporalis type bite, the more forward, the more masseter activation. 

Nobody wants this to happen to their head:

I have been jutting my jaw and pushing forward on my premaxilla, with a slight suction at the back of the palate. hoping to drag the maxilla forward. This activates my masseters with almost no temporal burn. With a more forward position of the jaws the muscles should engage more evenly, preventing excess head meat.

This thread should put light to the relationship between masticory muscles if you haven't already read it.

/community/community/main-forum/jaws-masseter-pattern-vs-temporal-pattern/