RxSwift, how do I chain different observables

I am still a beginner in Reactive programming, and RxSwift in general. I want to chain two different operation. In my case I simply want to download a zip file from a web server, and then unzip it locally. I also want, at the same time to show the progress of the downloaded files. So I started creating the first observable:

 class func rx_download(req:URLRequestConvertible, testId:String) -> Observable<Float> {

    let destination:Request.DownloadFileDestination = ...

    let obs:Observable<Float> = Observable.create { observer in
       let request =  Alamofire.download(req, destination: destination)
        request.progress { _, totalBytesWritten, totalBytesExpectedToWrite in
            if totalBytesExpectedToWrite > 0 {
                observer.onNext(Float(totalBytesWritten) / Float(totalBytesExpectedToWrite))
            }
            else {
                observer.onNext(0)
            }
        }
        request.response {  _, response, _, error in
            if let responseURL = response {
                if responseURL.statusCode == 200 {
                    observer.onNext(1.0)
                    observer.onCompleted()
                } else  {
                    let error = NSError(domain: "error", code: responseURL.statusCode, userInfo: nil)
                    observer.onError(error)
                }
            } else {
                let error = NSError(domain: "error", code: 500, userInfo: nil)
                observer.onError(error)
            }

            }
            return AnonymousDisposable () {
                request.cancel()
            }
        }
    return obs.retry(3)

}

After that, I create a similar function for the unzip

class func rx_unzip(testId:String) -> Observable<Float> {
    return Observable.create { observer in
        do {
            try Zip.unzipFile(NSURL.archivePath(testId), destination: NSURL.resourceDirectory(testId), overwrite: true, password: nil)
                {progress in
                    observer.onNext(Float(progress))
                }
        } catch let error {
            observer.onError(error)
        }
        observer.onCompleted()
        return NopDisposable.instance
    }
}

For now I have this logic on the "View model layer", so I download-> subscribe on completed-> unzip

What I want is to combine the two Observable in one, in order to perform the download first, then on completed unzip the file. Is there any way to do this?