Grunt usemin and useminPrepare multiple targets

From the usemin issues it appears that usemin and useminPrepare support multiple targets in the latest version:

Support multiple targets in useminPrepare:

• pull#162

• pull#206

usemin support:

• Multiple targets

I've tried using multiple targets with the following configuration:

useminPrepare: {
    foo: {
        dest: 'fooDist',
        src: ['foo/index.html']
    },
    bar: {
        dest: 'barDist',
        src: ['bar/index.html']
    }
},
usemin: {
    foo: {
        options: {
            assetsDirs : ['fooDist']
        },
        html: ['fooDist/**/*.html'],
        css: ['fooDist/styles/**/*.css']
    },
    bar: {
        options: {
            assetsDirs : ['barDist']
        },
        html: ['barDist/**/*.html'],
        css: ['barDist/styles/**/*.css']
    }
},

but I receive the following error:

Running "usemin:foo" (usemin) task Warning: Unsupported pattern: foo

Use --force to continue.

Using grunt-usemin 2.0.2

foo/index.html and bar/index.html being the main pages for 2 single page applications.

Thanks for your help!