Upload files to Firebase Storage RxJava2

Question

I'm trying to upload files to Firebase Storage with RxJava2. How do I make 1 method out of those 2? I mean how do I concatenate those 2 requests?

Not able to do flatMap. Anyone can help to refactor it?

 private fun uploadPhotos() {
    val urisList = globalStorageViewModel.urisList

    storageRepository.uploadFiles(urisList)
            .subscribeBy(
                    onNext = { downloadFileUrl(it) },
                    onError = { onError(it) }
            ).apply {
                disposables.add(this)
            }
}
private fun downloadFileUrl(it: Pair<UploadTask.TaskSnapshot, String>) {
    storageRepository.getDownloadUrl(it.first.storage)
            .subscribeBy(
                    onSuccess = { onFileUploaded(it) }
            ).apply {
                disposables.add(this)
            }
}