Typescript 'disable' excess property checking

Question

In typescript I have the following type:

type Base = {
  mandatoryProperty: number
}

Then I would like to create a Full type starting from Base that EITHER has a property1: number or property2: number, such as:

type Full = {
  mandatoryProperty: number
  property1: number
} | {
  mandatoryProperty: number
  property2: number
}

I create a Combined utility type:

type Combined<T, V> = {
  [K in keyof V]: {[R in keyof T]: T[R]} & { [P in K]: V[P] }
}[keyof V]

And lastly:

type Full = Combined<Base, {property1: number, property2: number}>

This type has the structure I expect it to have, yet, I can do something like this:

const X: Full = {
  mandatoryProperty: 3,
  property1: 2,
  property2: 4
}

I only want either property1 or property2 to be within the type. Not both. If I understand correctly, this occurs because of typescript's excess property checking behaviour. How can I force it to not occur?

EDIT:

As @jcalz mentioned, one way to achieve the desired behaviour is to set unwanted properties to never. I try doing so by changing the type to:

type Combined<T, V> = {
  [K in keyof V]: {[R in keyof T]: T[R]} & { [S in keyof V]: S extends K ? V[K] : never }
}[keyof V];

But this doesn't work. What is the reason?