sync two mpu9150s

In my application I use a microcontroller to communicate with two MPU9150s.
I use the DMP interrupt on both devices to specify when new data is available in the FIFO.
Because the MPU9150s are not running off of exactly the same clock, the time between the two interrupts varies over time. Thus I can't be sure that 0x68 will always interrupt before 0x69 with enough time between interrupts to perform necessary actions. Is there a way to sync the two MPU9150s occasionally to keep this timing more predictable?

I don't exactly understand the purpose of the FSYNC pin, but it doesn't seem to be relevant for this purpose.