Hard to give general advice without knowing the specifics. If one engineer insists on that everyone should use version control while another prefers to write to a shared active directory folder, then one engineer is right and the other is wrong. If someone insists on Pokemon exception handling (http://wiki.c2.com/?PokemonExceptionHandling) they are also wrong. But arguing over whether to use camel-case or snake-case for css classes is just pointless bike-shedding. If the engineers can't agree on that then there is probably some personal issues involved.

Maybe you can ask both engineers to create a document describing why their best practice is the best one? If one engineer spends his whole weekend writing a 30 page essay on why their best practice is the best and the other does not, let him (or her) have it.