ACMS is correct; there are two strobe units in each wingtip. BUT I would strongly advise NOT looking at them close up - you risk ruining your vision for a while which could be dangerous. Look at their light reflected on a nearby building instead.
The first strobe unit fires and immediately after that has happened the second one fires.
If one strobe unit in a wingtip fails, the other unit in the same wingtip will still flash, so you have built-in redundancy.
In the OP's case they said the left and right strobes were out of sync - not flashing together. I suspect the sync signal to one wing was absent; so that strobe unit was flashing asynchronously and it drifted away in timing from the one in the other wing, until it was flashing 180° out of phase.