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.