As for twitter, I would say just tweet them. If they haven't used twitter much they haven't changed their e-mail settings so will probably get an e-mail everytime someone mentions them in a tweet. They would probably see it and then the other person would just change their @username to something else and all their followers (if they have any) would go with them. And you could just take their old @username and all you tweets and followers carry over. Be sure to tell the other person that they don't have to delete their twitter and changing their username all there followers carry over.
As for instagram, I would I first look at the number of photos, you can see that whether it is locked or not, if it is more then 100 photos, I don't know if the person would want to change there username. But either way just request follow the person and hopefully they follow, then post a comment on one of their pictures explaining what you want to do. Again, if someone changes their username on instagram instead of creating a new account, all the followers and stuff transfers over.
