By default, the Video Player box use the Internal Sync Mode. In this mode, the clock of the videos are synced over the RealMotion master clock. Depending on your computer's performance, the Internal Mode allows the Video Player to drift of one frame or two to have a smooth playback.
When 2 videos need to be perfectly sync, use the External Sync Mode instead.
Go to the Properties of the Videos Player boxes -> Tab Advanced -> Sync Mode.
A Time input pin will be enabled. Connect this pin to the same box, like the Timer box.
The Video Player will then share the same clock, without drifting, and the videos will be perfectly sync.