why these two characters

Stoppards chooses this play (one of Shakespeare's most tragic) as a subject of comedy and "what could have beens." It's fun and entertaining. He directly quotes Shakespeare whenever possible, but he also completely re-writes the plot to serve his purpose of entertaining the reader. These two characters only had small roles in Hamlet; this allowed Stoppard to use a bit of liscense and make the characters his own. There weren't any great ideas or prior knowledge to interfere with who he wanted them to become in his own work.............