Sounds like you want RAID 0 for your boot drive. I'd recommend against that. If one drive fails the data on both drives is useless. You've essentially doubled your risk of data loss. Since you have NVMe which is already plenty fast, you'll not get any appreciable performance improvement either.
I don't know if it can actually be done (probably not), but I assume you'd have to backup your drive, then create a RAID and then restore the backup to the RAID array.