This is "old" news, but as I have noticed that there have been a lot of people join the forum in the last few years, I thought I would refresh the knowledge base on this.
The resistance of the zoom mechanism can put enough torque on the diopter adjustments of many binoviewrer that the diopter will turn. To prevent that, the natural inclination is to use one hand to zoom, and use the other to hold the diopter against rotation.
Here is the method I use when the zooms will be in the holders for a planetary session or other duration that you would consider more than a quick use.
The first step is to put the eyepieces in, make sure they are fully bottomed, then clamp them.
Next, bottom both diopters and tighten them hand tight against their bases. You do not need a lot of torque. You only need a little more than the torque required to turn the zoom.
Next, focus the telescope for each eye to determine the eyepiece that requires the most inward movement of the telescope focuser. Once you have determined the eye that takes the most inward travel, lock the telescope focuser so that it won't move.
Now, loosen the opposite eyepiece and while viewing though it, extract it slowly until you find the place where this eyepiece is in perfect focus. Usually this will only be a millimeter or two. Once you have it in focus, tighten the retaining screws or collar so that the eyepiece is locked into this adjusted focus position.
There you go, you are all set. When you need to zoom, you no longer have to worry about the diopter changing and you can use one hand on each eyepiece and zoom them together.
If the zooms are not parfocal, you will need to release the telescope focuser. It was only necessary to tighten it when doing the second eyepiece to ensure that the effort did not move the focuser for the first eyepiece.
Hope this helps someone today, or someday.