index.js

let main = new RenderPass(scene.root, camera)
let mainSave = new SaveSwapPass()
let mainRead = new TextureEffect({
	blendFunction: BlendFunction.NORMAL,
	texture: mainSave.renderTarget.texture
})

let composite = new EffectPass(camera,
	mainRead,
	// other inputs ...
	// other effects ...
	vignette)

let mainComposer = new EffectComposer(renderer)
let compositeComposer = new EffectComposer(renderer)

mainComposer.addPass(main)
mainComposer.addPass(mainSave)

compositeComposer.addPass(composite)

SaveSwapPass.js

import { SavePass } from 'postprocessing'

export class SaveSwapPass extends SavePass {
	render (renderer, inputBuffer, outputBuffer, deltaTime, stencilTest, composer) {
		let currentRenderTarget = this.renderTarget
		let composerRenderTargetName = inputBuffer === composer.inputBuffer
			? 'inputBuffer'
			: 'outputBuffer'

		this.renderTarget = composer[composerRenderTargetName]
		composer[composerRenderTargetName] = currentRenderTarget
	}
}