Is it ok to have an A and a B list for your wedding guests?

Yes, it’s totally fine. In fact, most people probably have a B list, if not actually written down somewhere then at least in their heads. While you should estimate that about 20% of invitees will be unable to attend your wedding, you also have to be careful not to over-invite and end up with more guests that you can handle. The answer to that dilemma is to have a second list of people you will invite if there is space after you start getting responses, as long as it is still several weeks before the wedding. 

References

Post, Peggy. Emily Post’s Wedding Etiquette. 5th ed. New York: HarperCollins, 2006. Print.

Haar, Amanda R. The Wedding Blue Book. Dalton: Crane, 2007. Print.