We have two Windows devices connected to Norton Family and I want to allow my daughter 2 hours screen time, but do not care how much of that time she spends on one vs the other. When I click "apply these rules to all devices child uses" it gives her 2 hours on each screen, for a total of 4 hours.
As per current design, Norton Family monitors time per device and not cumulative of all devices used by the child. As a workaround for your case, you can configure time limit on each device and restrict the time range to individual devices.
For example:
Device 1 - Number of hours allowed : 1 hour
Device 2 - Number of hours allowed : 1 hour
This way you can restrict the time limit across two devices.