I’m used to deal with ACF configurations and I think that the solution to your problem is quite simple, and is integrated in the ACF “gallery” field configuration options.
Check out this link, where you will find described all the options for a “gallery” field. As you will see, there’s a parameter “minimum selection” which description is:
The minimum number of images that are required for the field to
validate. Defaults to 0
As simple as writing ‘2’ in the configuration panel for this specific field, and ACF will check before saving if there are two or more images. If not, an alert message will appear.
I hope it helps!