No two are not necessary. Install one and it will use a PCIe 2 x 8 lane slot in a Mac Pro and give you a PCIe 3 x 4 lane at the M.2 adapter.
Sorry I cut and pasted the abbreviated links from earlier in this thread,. I'll correct, but the links are
Samsung 970 Evo SSD:
https://www.amazon.com/dp/B07BN217QG...ing=UTF8&psc=1
M.2 switched adapter card:
http://www.xlr8yourmac.com/feedback/...ro_Review.html
All you ever wanted to know about NVMe drives on a Mac Pro Cheesegrater:
https://forums.macrumors.com/threads...-ahci.2146725/
Slot availability matters becase you need a slot to put the adapter card in
But not just physical slot size, you want a slot with enough logical lanes actually available in it to meet the performance needs of the M.2 drive. i.e. you need 8 x PCIe 2 lanes for a single 920 Evo SSD, or 16 x PCIe 2 lanes for using two 970 Evo SSD on a single card... assuming you are using a switched card, otherwise you only need half those numbers and only get half the performance.
All this pre-supposed you are on Mojave, which gets you all you need including NVMe boot support.
Boot time sometimes comes up as an issue, and can increase as you add different drives, including NVMe. I hope it's not a issue for most users, and if it is you can reduce it with simpler disk configurations... going all NVMe as an example.
Here is another useful resource that does touch on boot time.
https://docs.google.com/document/d/1...h.2yfwj6riry79