Delay between two XBee End-Points when told to collect data from XBee Coordinator

Currently have 3 XBee S1 Modules in play.
One is set up as a Coordinator (API mode) and is connected to a laptop via a USB-adapter.
The other two are set up as End-Points (AT mode) and are connected to 2 Raspberry Pi’s Model 3 B.

Both RPi’s have an ADXL345 accelerometer attached to them in order to collect vibration data.
It’s important for our model that both RPi start collecting data at the same time (within 10 ms of eachother).

We have set up a Frame List in XCTU so we through our Coordinator can, via a python script on the RPi’s, start the collection of data.

Currently we are able to start collecting data within 0,5 seconds of eachother. So there is some kind of time stamp problem in our setup.
It seems like XBee End-Point 1 receives the given commands before XBee End-Point 2.

So our problem is that the data collected from node 1 always seem around 0,4-0,6 seconds ahead of node 2.
It happens both when we start and stop the data collection. So node 1 also stops 0,4-0,6 seconds before node 2 stops.

If needed I can provide more information. Just let me know!