How to mount a .sparsebundle after External Hard drive mounts
So, I found out a way to do what I wanted via, automator and shell scripts.

I made an Automator Folder Action. set the folder to /Volumes by pressing CMD+SHIFT+G and typing "/Volumes" and adding a Run AppleScript action. 

    on run {input, parameters}

set mountedVolumes to do shell script "ls /Volumes/"
if (mountedVolumes contains "External") is true then
    do shell script "hdiutil attach /Volumes/External/TimeMachine/TimeMachineTest.sparsebundle"
end if

return input


That way, when there is an update to the volumes folder, it runs the AppleScript, which mounts the .sparsebundle. 

edit:

Found a more simple way.

Create the Folder Action for /Volumes, then add Run Shell Script action.

  hdiutil mount /Volumes/External/TimeMachine/TimeMachineTest.sparsebundle