The difference between nSeqs happens because the actual recording only starts when all devices have started acquiring data.
Generally there are a few milliseconds delay between them, and in this case data from the BITalino with nSeq 0 was the last to start arriving, with nSeq 4 was the second and with nSeq 11 was the first.
In order to synchronize the devices you need to have a common marker triggered between them. This can be done with anything ranging from light sensors on two of the devices and an LED on the other, or with a synchronization cable connected between digital ports.
With this setup whenever you activate a digital output on the "master" device, this will be marked on the data from that device as an activation of the digital output and on the receivers as a change on a digital input pin or amplitude level of the analog signal (in case of the light sensor approach).
I hope you found this information useful. If you have further questions, let us know. To help in this process we have a sync cable on our store:https://store.plux.info/cables/231-synchronization-cable.html
The BITalino Team