I'm currently running an ASUS Prime Z390 with a 1TB Samsung Evo Plus in the top M.2 slot.
My question is if I would like to add an additional M.2 drive, should it be the same make, model, and size?
Thanks for the question! As you may or may not know, DRAM has additional settings that affect its performance like timings and voltage.
When you mix different speeds of DRAM, JEDEC settings will allow the sticks to work together in most cases with a default timing and voltage setup with the downside being you won't be able to run XMP or DOCP. This is why many will recommend you buy your memory in "kit" form.
With M.2 SSDs, you don't necessarily need to worry too much about matching drives, unless you are planning to setup a glorious RAID array with your EVO+. I would, however, stick to known brands with a solid warranty like the Samsung, Western Digital, and SanDisk solutions have.
One thing you do need to pay attention to is your motherboard's M.2 configuration and in your case, your bottom M.2 slot comes off the Z390 chipset and supports both SATA and NVMe M.2 drives.