Req some help with connecting external hard drives up to MacBook Pro.

Hi, this is the first time I've needed to post on here after owning Apple products for some time. I can't get my head around the best way to do something and I'm hoping someone can point me in the right direction.

I have a MacBook Pro Mid 2015. I have an external hard drive (WD My Passport for Mac), which I have partitioned - one part for Time Machine back up and the other for extra storage. I need to back up the files in the 'storage' part of this drive and realise I will need an extra hard drive. This is where I am getting myself confused!

What is the best way to go about this? Do I need to get an extra hard drive and connect it up via the other USB port on my MacBook, do I need a USB hub so I still have a spare USB port on my Mac or can I somehow piggyback the 2 hard drives? When I come to back up do I just set up a Time Machine Back up just on the 'Storage' part of my hard drive?

Any help / advice on this would be much appreciated I just can't work out the best way to do it.

Thanks