I don't expect a scenario where Haaland and Mbappe are on the same team in the future. This is not a good choice for any team because after all they are both competitors in the current era. Both are productive center forwards, but I prefer Haaland as a pure center forward over Mbappe.
It will not be a bad decision for any team that will have them both, it will only affect the players individual goals because any of them can score. I love them both because of their goal scoring mentality, if both of them can be in Madrid team, they can win any trophy just that they are both big too good defensively.
It's not a bad scheme but on the other hand it is difficult to do and might even sound quite impossible to do.