I was wondering what the consequences were at school for the behaviour. That might be one of the reasons for using at "home" is because they can't at school. Knowing what they do at school about it might help to give you some ideas of how to have similar consequences outside of school.

They only do it if they have an audience so put up two mirrors in out of the way spaces and send one to each mirror and say there you can be your own audience and make them stand there and talk to themselves till you let them go - a variation of sending them to the corner for timeout. At first it will seem like a joke to them but after the third or fourth time of you pointing and saying "to the mirror now" it will get old and reinforces that you are serious about there being non of it in play.