How to concatenate two streams using gulp?

For example I have a task with next content:

function task() {
    gulp.src('libs/*.js')
        // some manipulations
        .concat('libs.js')

    gulp.src('js/*.js')
        // Another manipulations
        .concat('app.js')
}

What if I don't want to put this files anywhere in file system? Can I somehow concatenate libs.js and app.js inside a task?