One issue is the same intervention doesn't necessarily affect both. For example, where I live West Nile virus is transmitted primarily by Culex pippiens mosquitoes, while the most abundant nuisance mosquito is Ochlerotatus stimulans.

Controlling one species will not greatly affect the other (they breed in radically different conditions). It's not a matter of scaling up operations; you need an entirely different strategy, with commensurate increase in operating costs, complexity, potential failure points, etc etc.

Give me unlimited resources and global remit and I'll take them all out, absent this prioritisation becomes necessary.