[tonester]

Instead of using the Secondary Driver feature/option, perhaps try this method (you might want to delete the Secondary Driver profile before attempting this):

-------------
After downloading the Kia Access app, the second driver has to register an account with Kia.

When Kia asks second driver to input VIN, click skip

First driver then opens Kia Access app and clicks [ … MORE], [Manage all vehicles], [Additional Drivers], [Add Driver]

Input second driver’s email address that was just registered; agree to all sharing

Second driver will see a button appear in app to accept sharing.

It will display “Link in Progress” in second driver’s app, even after stating link confirmed in first driver’s app. This remains even after second driver’s app and phone have been restarted.

Second driver opens app, logs out of app, then logs back in. “Link Confirmed” Both drivers now have app access to the EV6.
-------

Try doing that and see if it allows for both drivers to receive notifications. I did read where OP indicated that his wife did create her own account but may want to see if she did the second step (skip the input VIN request). Note that this method does not require you to use/set up the "Add Secondary Driver" feature in the Kia Access app.