First of all, we should note that our early Church Fathers (Origen and Clement of Alexandria) did not recognize Mark 16:9-20 as being in the early manuscripts. Modern theologians agree and point out the fact that Matthew, Luke and John do not include these verses. There are Greek words and expressions that are foreign to Marks writings, and it is surmised that someone added these verses later. Scholars say that these verses, when being used or preached, people should use caution in quoting and instructing from 9-20.
At best, the addition of the word Baptize should pale when considering the word "believe". In verse 16, the word baptize does not appear in the last statement. If this verse is valid, Jesus is merely saying that belief and baptism is the important formula, if you will, in the course of Salvation. Belief is a silent statement to Jesus and His Father coupled with the first "work", baptism. There was no need to include the word baptize if someone refused to believe.