I have been doing some study on baptism in the New Testament, and I came across the following interesting difference. The Douay Version (with Vulgate) of Mt. 20:22-23 reads thus:
 And Jesus answering, said: You know not what you ask. Can you drink the chalice that I shall drink? They say to him: We can.
Respondens autem Jesus, dixit: Nescitis quid petatis. Potestis bibere calicem, quem ego bibiturus sum? Dicunt ei: Possumus.
 He saith to them: My chalice indeed you shall drink; but to sit on my right or left hand, is not mine to give to you, but to them for whom it is prepared by my Father.
Ait illis: Calicem quidem meum bibetis: sedere autem ad dexteram meam vel sinistram non est meum dare vobis, sed quibus paratum est a Patre meo.
However, the King James adds to the text:
 But Jesus answered and said, Ye know not what ye ask. Are ye able to drink of the cup that I shall drink of, and to be baptized with the baptism that I am baptized with? They say unto him, We are able.
 And he saith unto them, Ye shall drink indeed of my cup, and be baptized with the baptism that I am baptized with: but to sit on my right hand, and on my left, is not mine to give, but it shall be given to them for whom it is prepared of my Father.
It appears that the KJV translators gratuitously added details from Mark 10:
 And Jesus said to them: You know not what you ask. Can you drink of the chalice that I drink of: or be baptized with the baptism wherewith I am baptized?
 But they said to him: We can. And Jesus saith to them: You shall indeed drink of the chalice that I drink of: and with the baptism wherewith I am baptized, you shall be baptized.
So my question is, does anyone know anything about the thinking of the KJV translators and why they might have done what appears to be a joining together of verses, let’s be gracious and say for clarity’s sake, thus adding to what is not in the original?