Do you really think there's much expense to having two code bases for simple apps (as you said) like Instagram?
If these apps are written properly most (all) of any custom logic is done server side and they are just presentational only.
In addition, a good developer/engineer/team that's properly focused on mobile (the size of the team of Instagram) should have absolutely no problem supporting two native codebases.
Think of it as not 2, but 3 different languages. Say your backend is node - your JavaScript developers can all work across the stack. You can keep code standards more or less the same across the whole company.
If these apps are written properly most (all) of any custom logic is done server side and they are just presentational only.
In addition, a good developer/engineer/team that's properly focused on mobile (the size of the team of Instagram) should have absolutely no problem supporting two native codebases.