I don't think Mercedes would risk the fireworks of Lewis vs Max. It would be good for the spectator though in my opinion. It would stop each race being Hamilton win as the default position, and we probably would get some fireworks. It would be especially interesting if we get to 2021 and another team has stolen a march on Mercedes under the new regulations.
If I could guarantee that Red Bull, Ferrari and Mercedes would have similar performance for a season then my preference would be more towards keeping Max in one team, Hamilton in another team, and Vettel and Leclerc in another.