@Hazoss,
Well done, congratulations.
You can't backup (BU) simultaneously; but what you can do, — and indeed the OS is designed for this — is connect both EHDs to your iMac and use the Time Machine icon within System Preferences to set up both EHDs as Time Machine drives.
Then what happens is that the Time Machine BU alternates between the two EHDs; in the background and with no need of thought or input from you.
(Additionally, at any time between BUs, if you feel the need, you can click on the Time Machine icon in the top menu bar and click on "Back Up Now" from the dropdown).
Come back if any of that is unclear or need any sort of help.
Ian