You must know what I mean? When your students play their “hands-together” scales, and the fingers of each hand aren’t coordinated, so that the notes don’t sound together at the same time?
The solution to so many technical problems is usually to go slower, so that you have time to pay close attention to what you are doing. In the case of kerplunking in scales, the chances are that the finger of one hand is ready to play a note before the finger on the other hand has arrived on its note. The delay can be caused by having to pass the fingers or thumb over each other, or maybe by the interruption to the flow of the fingers due to moving from black to white notes (or vice-versa).
Try playing the scale slowly enough that you can actually touch each note before you play it. Do this a few times, and then gradually go a little faster, listening with acute concentration to make sure that the notes sound exactly together. You need to be super-sure that you are transferring cleanly from finger to the next, so that you don’t start holding notes down and creating a blurred sound.