Many cameras do not record EXIF sub-second timestamps, and if a camera doesn’t write that field, there usually isn’t a setting to add it later in-camera.

A simple, reliable alternative is to include an auto-incrementing sequence in the filename rather than relying only on capture time. For example:

yyyyMMdd_cameraCode_###

or combine date/time with a sequence:

yyyyMMdd_hhmmss_cameraCode_###

That avoids collisions even when multiple images are taken in the same second, whether from the same camera or different cameras. Adding a camera identifier is also helpful when importing from several devices into one library.

So the practical solution is: keep the date/time for readability, but add a unique suffix such as an incrementing number.