If you have two such people, you might get there by making each of them the sole maintainer of the other's code. They might - might - see the problems when it's being done to them, instead of them doing it to others.

More in general, the problem is that these people have been in an environment where programs were small enough that those practices didn't cause problems. They've moved into an environment where the scale is different, and those practices now cause real pain. Until they experience the pain (rather than just being told about it), they aren't really going to get it.