And having both is not really 1.5x to 2x the work, in my experience. Maybe more like 25% to 30%. A lot of components and widgets can be reused, the typography and colors can be similar, and certain screens can keep their layout with just small tweaks.
Sure, maybe the initial design takes a bit longer since each screen needs several breakpoints. But that's only a small part of the overall design work anyway. Then on an ongoing basis, your previously established patterns (and components in code) can largely be reused with small tweaks, easily done with modern toolkits like Tailwind or MUI.
I don't think it's that big a deal. Web devs have been doing mobile designs for more than a decade now, and the tools have gotten better and better. Honestly, it's way less wasteful than Agile ceremonies or endless meetings. If you want to stay lean and cut cruft, take it from places that don't directly affect the user, not the one place where they actually use your product all day.
That's what you tell your customers at least :)
If anything, having two designs that look and function well is less work than having one design that looks and functions well on two disparate platforms with completely different affordances.
How do you reconcile this with the fact that you need almost double the effort for 2 separate designs? I'd say you need at least 2x of Designer work and at least 1.5x of front-end Dev work to achieve this.
Where will additional resources to support this come from?