I’m on my 3rd System76 Gazelle. I’ve ordered them with a single drive, and added a drive when needed. I have a 2.5" spinning rust drive I bought a while back and ran it in my last Gazelle, but haven’t put it in my new Gazelle, yet. The internal NVMe drive is super fast. The 2.5" drive is a hybrid drive. I can add another NVMe, I think.
IF you’re looking for higher read speeds, you can use something like LVM to add both NVMe drives to a Volume Group and have LVM use RAID1 type mirror for your Logical Volumes. This allows LVM to read half your data from each drive, theoretically doubling your drive throughput on reads. I propose LVM for this solution as it is highly flexible opposed to using mdadm.
Also keep in mind that you want to leave plenty of free space on your flash storage to give it room to perform wear leveling on writes. Fortunately, a side-effect of running Arch is that most of the files on the system are replaced much more often than other Linux distros, so this helps with wear leveling.
Keeping your OS on a separate drive from your data can be an advantage in OS-reinstall and recovery scenarios. Keeping /home on a separate drive allows you to nuke and pave much more easily, if you’re that kind of Linux user. I usually start off with /home on the main partition (LVM logical volume) and wind up moving it elsewhere down the road.
Another advantage of having a separate drive/partition is that you can use LUKs to encrypt it, and still be able to do updates and reboots without needing to enter the encryption key. You only enter the key when you need to actually mount the encrypted drive/partition. For a system that might be remote, this has advantages. If you fully encrypt your system, doing a remote reboot has challenges.