Sachs has the right idea, they need to do something where they have no choice but to rely on each other - can you send them of camping somewhere remote - give one the tins of beans and the other the can opener, one the tent and the other the poles etc - they will then have no option other than to work together or they freeze and starve.
Other than that, you need to seperate them - tell them that you can only keep one of them and you will be judging them over the next week in order to decide which one!