Calling these tabs is confusing the issue. They're just forms that happen to be in a tabbed layout. Trying to do some type of validation before the customer clicks from one form (tab) to the other form (tab) by using the Activate/Deactivate events will lead to problems in my opinion. You may end up actually having to use the Activate event of the first form to run your check and set focus back to the second form, but this would be a terrible idea because if the first form is ever opened without the second form being open then this would cause an error. Even if both forms are open, if something goes wrong the customer may find it impossible to set focus back to the first form at all.
If you want to force a customer to remain on a given form until certain circumstances are met, then it should be opened in Dialog mode and you should use an event that can be reliably cancelled, like Before_Update or Form_Unload, to do your validation. Personally, I wouldn't use tabbed layout for forms, and definitely not in a situation like yours where you need to control when and how a customer leaves the form.