Bash – Add second sub-key to unattended GPG key

I am writing a bash script which generates unattended GPG keys, I have looked through Unattended Usage of GPG and was surprised to find that 'Currently only one subkey can be handled.'

I am unable to find if there is a way to modify a GPG key to add a second subkey using the unattended generation functions available, or if I'll have to add the subkey manually myself.

I have attempted to use here documents, here strings and creating a file with one input per line as well as using printf similar to below:

printf 'addkey' | gpg2 --edit-key 'test@test.com'

With none of these solutions working:

balthasar@magi:~$ printf 'addkey' | gpg2 --edit-key 'test3@test.com'
printf 'addkey' | gpg2 --edit-key 'test3@test.com'
gpg (GnuPG) 2.0.26; Copyright (C) 2013 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  4096R/AB96CED4  created: 2016-12-02  expires: 2017-09-02  usage: SC  
                     trust: ultimate      validity: ultimate
sub  2048R/71804CF2  created: 2016-12-02  expires: 2017-09-02  usage: E   
[ultimate] (1). test3 <test3@test.com>

gpg> 

My GPG Key File

Key-Type: RSA
Key-Length: 4096
Key-Usage: sign
Subkey-Type: RSA
Subkey-Length: 2048
Subkey-Usage: encrypt
Name-Real: Foo Bar
Name-Email: foobar@test.com
Expire-Date: 2017-09-02
%ask-passphrase
%commit

UPDATE: 4, December 2016

I have passed the first hurdle of getting some input to GPG by using the following snippet:

~$ stty -echo; gpg2 --edit-key <keyname> "addkey"

However I am unable to 'pipe' input after running the addkey command.