That slot is not NVMe natively, so you can't boot from that M.2 slot. You have to mod the BIOS and insert a little NVMe support module and reflash, you can find the guide here or on Win-Raid forum.
Anyway you can use all the slots, removing the case of the SATA 2.5 SSD and using only the internal PCB. In the photo is mine.
But I used the stock 128 GB SSD that has a small PCB, so it doesn't touch the M.2 drive I installed later. With a 1 TB 860 EVO, probably they will touch.
If you want that large SSD, you have to do something like this: https://rog.asus.com/forum/showthread.php?57888-G751JY-Replace-M2-with-2-5-quot-Caddy#post476675
It's not a very clean solution in my opinion, and probably you will see higher temperatures on the M.2 drive because of the tape coverage and poor cooling. You can try to use as less tape as possible, just to keep SSDs in place but not cover them up too much.
Thank you. Can I just put the stock silicon pads between them to prevent touching? The ones that came with the m.2 ssd?
The problem is that the M.2 PCB and the 2.5 Sata PCB are at the same height. So you have to tilt them a bit, in the linked thread it seems that the M.2 is a bit bend downward, and the Sata 2.5 a bit upward.
Since this, I think you can use silicon pads between them, but you have also to use some tape to keep the Sata 2.5 in place, because the M.2 under it will push the Sata upward.