As far as I know, there is no restriction against it. They can both be a “best man”. My fiance has a maid of honor AND a matron of honor lined up for our wedding.
(Oy! And they say Yanks murder the English Language!)
I believe canon law only calls for “two witnesses” and does not specify their sex, so you could have two male witnesses. Otherwise, it is up to you and your beloved which of your attendants will serve as witnesses.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.