If the current clowns are both over three years old, and have been in the same tank that same time, and they do not already host together, I would suspect they may never... you may have two females who are just 'putting up with each other', if there is enough room in the tank for them to have their own territories staked out.
If it was my tank, rather than trying to get two fish to do something they are not doing naturally, I would trade one for a new juvenile, and you will likely have a much better chance of them pairing up and making babies, eventually... but if you go this route... you will need to remove both current fish, re-aquascape a bit to change any existing territorial boundaries with good hiding spots, then add the new juvi to the tank first to let him get acclimated and a good a lay of the land. After a few weeks, re-add one of your current clowns back and monitor closely.
