The metering flashes are popped off one at a time before the shutter is activated for the exposure. The sequence is so rapid that it looks to our eyes like a singular pop, but it isn't. I don't know specifically about Olympus/Panasonic, but in general most TTL systems use a fairly low power pulse to measure the reflectivity of things in the scene. This preserves as much as is possible of the energy loaded into the flash's capacitors for the actual exposure.

When using off-camera flashes with systems that use optical control (instead of radio), there is a second pre-flash (it's actually an encoded pulse) that tells each group what power to use and gives a timing instruction letting them know how long to delay until the flash is fired for the exposure. The system then times the shutter release to be in sync with when the flashes have been told to fire.

Once the camera has metered each pre-flash, calculated the power needed from each group, and transmitted instructions to each group the shutter (whether mechanical or electronic) opens and all flashes pop at the same time.