Email people who registered but did not attend a webinar?

It will depend on how you have stored the data.  Do you have separate static lists for those that attended webinar A and registered for Webinar A and same for Webinar B. 

And/Or do you have a program set up for Webinar A and they are in specific program statuses.  Best practices should have the people who registered but did not attend in a status of No Show, and the ones that attended be in a status of Attended.  

In the Smart List for the smart campaign you can do the following.

If scenario 1)  Member of list = Registered Webinar A, Registered Webinar B. Not member of list Attended Webinar A, Attended Webinar B

If scenario 2) member of program is webinar A, webinar B, constrain by status is No Show.