Backing up additional hard drives with EX2 Ultra

My Cloud OS 3 Personal & Network Attached Storage My Cloud EX2
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Hi! Hope this post finds you well.
I’m a newbie so apologies if this is a silly question.
A bit of context. I’m on an iMac and I am using my EX2 Ultra (8TB) with Time Machine to back it up. I’ve got it set up so that there is 4TB of capacity (presumably the other 4TB is backing up the first drive!).
I also have a MyCloud Home which I’m using as a Plex Server.
My questions is, how can I backup my MyCloud Home onto my EX2 Ultra (as well as keeping it as the Time Machine destination for the iMac)?
Thanks for your time and consideration.