Once you have told me your configuation?

It is vital if you run two screens that both screens are set to the same refresh rate or frequency.
This depends on what screens you are using, and what kindof outputs you are using

For 30fps playback, both screens must be set to 60Hz.
For 25fps playback, both screens must be set to 75Hz.

You must not be using any kindof apple studio display, as the refresh rate on these types of monitors cannot be changed.