Work Laptop - Boot drive in PCI-E x2 or x4?

Getting a chance add some hardware to a new work laptop
Lenovo P52, it came with a little 256gb ssd in one of the M.2 slots
It’s not even close to enough space.
Luckily the thing has another M.2 slot, but the specs says this is a PCI-E 3.0 x2 slot…

If this a lie (refering to the optane being x2 and not the slot) then the following questions are moot

I plan on simply adding an extra M.2 ssd to the 2nd slot (1tb, probably an 970 evo)
Question is should i put the boot drive in the x2 slot? or this 2nd drive?
Use cases:
Most of my work is going to run off the 2nd drive. Lots of time spent searching text in tons of tiny files.
I do run programs that are going to be stored on the OS drive, but I will also be running some programs off the 2nd drive
My current laptop only has 1 sata ssd for os and data (partitioned) so no matter what I’m going to get a performance boost.

inb4 put the OS on a 2.5in sata drive and go raid 1 with 2x 1tb M.2 drives.
I would but the boss would look at me funny on why I suddenly need 3 drives purchased instead of 1.

I’ve never heard of a x2 before, but even if it is, is that such a big deal?

Pcie 3.0 x1 is 985MB/s, so x2 should be 1970MB/s. Considering that most people are working with smaller files and aren’t able to take full advantage of nvme and 4k random read/write is often well below the theoretical 1970, I think it basically doesn’t matter. The only time you would be bottlenecked is if you were working with only large files that could be read or written sequentially.