There is a difference between that online scan type and the installed regular anti virus programs.
The Norton {installed type} runs in the background to catch anything suspicious before it can do damage.
Or it should be doing that if the settings are set that way.

And there are levels of programs also - basic - AV only, AV & Anti spyware, AV/ anti spy & firewall, and then the internet suites or extreme type programs that cover just about everything.

so depending on what Norton type they have...they could be covered well or not so well.


But back to the having 2 or more AV etc at once -

you can have them installed but only have 1 running actively at a time.
then use the others as one time scans to double check for baddies.
So the main thing is not to have 2 or more actively running full time at the same time.
Hope that explains it for you.

I have Zone Alarm extreme running as main full time protection.
but I also have malwarebytes, super anti spyware and some others to run as stand alone scans, but they aren't active until I run the scan myself.


for now I would just have them wait and see if the notice pops up again or not...