error TS5053: Option 'declarationDir' cannot be specified with option 'outFile'.

Attempting to change the declaration file output directory, I get the following error:

error TS5053: Option 'declarationDir' cannot be specified with option 'outFile'.

tsconfig.json looks like this:

{
    "$schema": "http://json.schemastore.org/tsconfig",
    "compilerOptions": {
        "module": "amd",
        "target": "es6",
        "outFile": "../assets/js/main.js",
        "sourceMap": true,
        "declaration": true,
        "declarationDir": "../"
    }
}

I want my .d.ts file emitted somewhere else, since that's not a public asset.

Why is this not permitted?

I'm forced to create two (near-identical) tsconfig files and run the compiler twice to get the declaration file and output file. What for?