its an obvious question you are asking. sure if both kids were at the same level then you bring up the kid that plays that position. but if you have a really good player at one position and he is more skilled than the other jv player then yes you bring him up. Also there are many players in the NFL that have been wide receivers in college and then switched over to defensive back because the coaches see the athletic ability and feel he is better than the other defensive backs that were at the tryouts.
